msgwriter_test.go

// SPDX-FileCopyrightText: The go-mail Authors
//
// SPDX-License-Identifier: MIT

package mail

import (
	"bytes"
	"crypto/rand"
	"errors"
	"fmt"
	"io"
	"mime"
	"runtime"
	"strings"
	"testing"
	"time"
)

func TestMsgWriter_Write(t *testing.T) {
	t.Run("msgWriter writes to memory for all charsets", func(t *testing.T) {
		for _, tt := range charsetTests {
			t.Run(tt.name, func(t *testing.T) {
				buffer := bytes.NewBuffer(nil)
				msgwriter := &msgWriter{
					writer:  buffer,
					charset: tt.value,
					encoder: mime.QEncoding,
				}
				_, err := msgwriter.Write([]byte("test"))
				if err != nil {
					t.Errorf("msgWriter failed to write: %s", err)
				}
			})
		}
	})
	t.Run("msgWriter writes to memory for all encodings", func(t *testing.T) {
		for _, tt := range encodingTests {
			t.Run(tt.name, func(t *testing.T) {
				buffer := bytes.NewBuffer(nil)
				msgwriter := &msgWriter{
					writer:  buffer,
					charset: CharsetUTF8,
					encoder: getEncoder(tt.value),
				}
				_, err := msgwriter.Write([]byte("test"))
				if err != nil {
					t.Errorf("msgWriter failed to write: %s", err)
				}
			})
		}
	})
	t.Run("msgWriter should fail on write", func(t *testing.T) {
		msgwriter := &msgWriter{
			writer:  failReadWriteSeekCloser{},
			charset: CharsetUTF8,
			encoder: getEncoder(EncodingQP),
		}
		_, err := msgwriter.Write([]byte("test"))
		if err == nil {
			t.Fatalf("msgWriter was supposed to fail on write")
		}
	})
	t.Run("msgWriter should fail on previous error", func(t *testing.T) {
		buffer := bytes.NewBuffer(nil)
		msgwriter := &msgWriter{
			writer:  buffer,
			charset: CharsetUTF8,
			encoder: getEncoder(EncodingQP),
		}
		_, err := msgwriter.Write([]byte("test"))
		if err != nil {
			t.Errorf("msgWriter failed to write: %s", err)
		}
		msgwriter.err = errors.New("intentionally failed")
		_, err = msgwriter.Write([]byte("test2"))
		if err == nil {
			t.Fatalf("msgWriter was supposed to fail on second write")
		}
	})
}

func TestMsgWriter_writeMsg(t *testing.T) {
	msgwriter := &msgWriter{
		charset: CharsetUTF8,
		encoder: getEncoder(EncodingQP),
	}
	t.Run("msgWriter writes a simple message", func(t *testing.T) {
		buffer := bytes.NewBuffer(nil)
		now := time.Now()
		msgwriter.writer = buffer
		message := testMessage(t)
		message.SetDateWithValue(now)
		message.SetMessageIDWithValue("message@id.com")
		message.SetBulk()
		msgwriter.writeMsg(message)
		if msgwriter.err != nil {
			t.Errorf("msgWriter failed to write: %s", msgwriter.err)
		}

		var incorrectFields []string
		if !strings.Contains(buffer.String(), "MIME-Version: 1.0\r\n") {
			incorrectFields = append(incorrectFields, "MIME-Version")
		}
		if !strings.Contains(buffer.String(), fmt.Sprintf("Date: %s\r\n", now.Format(time.RFC1123Z))) {
			incorrectFields = append(incorrectFields, "Date")
		}
		if !strings.Contains(buffer.String(), "Message-ID: <message@id.com>\r\n") {
			incorrectFields = append(incorrectFields, "Message-ID")
		}
		if !strings.Contains(buffer.String(), "Precedence: bulk\r\n") {
			incorrectFields = append(incorrectFields, "Precedence")
		}
		if !strings.Contains(buffer.String(), "X-Auto-Response-Suppress: All\r\n") {
			incorrectFields = append(incorrectFields, "X-Auto-Response-Suppress")
		}
		if !strings.Contains(buffer.String(), "Subject: Testmail\r\n") {
			incorrectFields = append(incorrectFields, "Subject")
		}
		if !strings.Contains(buffer.String(), "User-Agent: go-mail v") {
			incorrectFields = append(incorrectFields, "User-Agent")
		}
		if !strings.Contains(buffer.String(), "X-Mailer: go-mail v") {
			incorrectFields = append(incorrectFields, "X-Mailer")
		}
		if !strings.Contains(buffer.String(), `From: <`+TestSenderValid+`>`) {
			incorrectFields = append(incorrectFields, "From")
		}
		if !strings.Contains(buffer.String(), `To: <`+TestRcptValid+`>`) {
			incorrectFields = append(incorrectFields, "From")
		}
		if !strings.Contains(buffer.String(), "Content-Type: text/plain; charset=UTF-8\r\n") {
			incorrectFields = append(incorrectFields, "Content-Type")
		}
		if !strings.Contains(buffer.String(), "Content-Transfer-Encoding: quoted-printable\r\n") {
			incorrectFields = append(incorrectFields, "Content-Transfer-Encoding")
		}
		if !strings.HasSuffix(buffer.String(), "\r\n\r\nTestmail") {
			incorrectFields = append(incorrectFields, "Message body")
		}
		if len(incorrectFields) > 0 {
			t.Fatalf("msgWriter failed to write correct fields: %s - mail: %s",
				strings.Join(incorrectFields, ", "), buffer.String())
		}
	})
	t.Run("msgWriter with no from address uses envelope from", func(t *testing.T) {
		buffer := bytes.NewBuffer(nil)
		msgwriter.writer = buffer
		message := NewMsg()
		if message == nil {
			t.Fatal("failed to create new message")
		}
		if err := message.EnvelopeFrom(TestSenderValid); err != nil {
			t.Errorf("failed to set sender address: %s", err)
		}
		if err := message.To(TestRcptValid); err != nil {
			t.Errorf("failed to set recipient address: %s", err)
		}
		message.Subject("Testmail")
		message.SetBodyString(TypeTextPlain, "Testmail")
		msgwriter.writeMsg(message)
		if msgwriter.err != nil {
			t.Errorf("msgWriter failed to write: %s", msgwriter.err)
		}
		if !strings.Contains(buffer.String(), "From: <"+TestSenderValid+">") {
			t.Errorf("expected envelope from address as from address, got: %s", buffer.String())
		}
	})
	t.Run("msgWriter with no from address or envelope from", func(t *testing.T) {
		buffer := bytes.NewBuffer(nil)
		msgwriter.writer = buffer
		message := NewMsg()
		if message == nil {
			t.Fatal("failed to create new message")
		}
		msgwriter.writeMsg(message)
		if msgwriter.err != nil {
			t.Errorf("msgWriter failed to write: %s", msgwriter.err)
		}
		if strings.Contains(buffer.String(), "From:") {
			t.Errorf("expected no from address, got: %s", buffer.String())
		}
	})
	t.Run("msgWriter writes a multipart/mixed message", func(t *testing.T) {
		buffer := bytes.NewBuffer(nil)
		msgwriter.writer = buffer
		message := testMessage(t, WithBoundary("testboundary"))
		message.AttachFile("testdata/attachment.txt")
		msgwriter.writeMsg(message)
		if msgwriter.err != nil {
			t.Errorf("msgWriter failed to write: %s", msgwriter.err)
		}
		if !strings.Contains(buffer.String(), "Content-Type: multipart/mixed") {
			t.Errorf("expected multipart/mixed, got: %s", buffer.String())
		}
		if !strings.Contains(buffer.String(), "--testboundary\r\n") {
			t.Errorf("expected boundary, got: %s", buffer.String())
		}
		if !strings.Contains(buffer.String(), "--testboundary--") {
			t.Errorf("expected end boundary, got: %s", buffer.String())
		}
	})
	t.Run("msgWriter writes a multipart/related message", func(t *testing.T) {
		buffer := bytes.NewBuffer(nil)
		msgwriter.writer = buffer
		message := testMessage(t, WithBoundary("testboundary"))
		message.EmbedFile("testdata/embed.txt")
		msgwriter.writeMsg(message)
		if msgwriter.err != nil {
			t.Errorf("msgWriter failed to write: %s", msgwriter.err)
		}
		if !strings.Contains(buffer.String(), "Content-Type: multipart/related") {
			t.Errorf("expected multipart/related, got: %s", buffer.String())
		}
		if !strings.Contains(buffer.String(), "--testboundary\r\n") {
			t.Errorf("expected boundary, got: %s", buffer.String())
		}
		if !strings.Contains(buffer.String(), "--testboundary--") {
			t.Errorf("expected end boundary, got: %s", buffer.String())
		}
	})
	t.Run("msgWriter writes a multipart/alternative message", func(t *testing.T) {
		buffer := bytes.NewBuffer(nil)
		msgwriter.writer = buffer
		message := testMessage(t, WithBoundary("testboundary"))
		message.AddAlternativeString(TypeTextHTML, "<html><body><h1>Testmail</h1></body></html>")
		msgwriter.writeMsg(message)
		if msgwriter.err != nil {
			t.Errorf("msgWriter failed to write: %s", msgwriter.err)
		}
		if !strings.Contains(buffer.String(), "Content-Type: multipart/alternative") {
			t.Errorf("expected multipart/alternative, got: %s", buffer.String())
		}
		if !strings.Contains(buffer.String(), "--testboundary\r\n") {
			t.Errorf("expected boundary, got: %s", buffer.String())
		}
		if !strings.Contains(buffer.String(), "--testboundary--") {
			t.Errorf("expected end boundary, got: %s", buffer.String())
		}
	})
	t.Run("msgWriter writes a application/pgp-encrypted message", func(t *testing.T) {
		buffer := bytes.NewBuffer(nil)
		msgwriter.writer = buffer
		message := testMessage(t, WithPGPType(PGPEncrypt), WithBoundary("testboundary"))
		msgwriter.writeMsg(message)
		if msgwriter.err != nil {
			t.Errorf("msgWriter failed to write: %s", msgwriter.err)
		}
		if !strings.Contains(buffer.String(), "Content-Type: multipart/encrypted") {
			t.Errorf("expected multipart/encrypted, got: %s", buffer.String())
		}
		if !strings.Contains(buffer.String(), "--testboundary\r\n") {
			t.Errorf("expected boundary, got: %s", buffer.String())
		}
	})
	t.Run("msgWriter writes a application/pgp-signature message", func(t *testing.T) {
		buffer := bytes.NewBuffer(nil)
		msgwriter.writer = buffer
		message := testMessage(t, WithPGPType(PGPSignature), WithBoundary("testboundary"))
		msgwriter.writeMsg(message)
		if msgwriter.err != nil {
			t.Errorf("msgWriter failed to write: %s", msgwriter.err)
		}
		if !strings.Contains(buffer.String(), "Content-Type: multipart/signed") {
			t.Errorf("expected multipart/signed, got: %s", buffer.String())
		}
		if !strings.Contains(buffer.String(), "--testboundary\r\n") {
			t.Errorf("expected boundary, got: %s", buffer.String())
		}
	})
	t.Run("msgWriter should ignore NoPGP", func(t *testing.T) {
		buffer := bytes.NewBuffer(nil)
		msgwriter.writer = buffer
		message := testMessage(t, WithBoundary("testboundary"))
		message.pgptype = 9
		msgwriter.writeMsg(message)
		if msgwriter.err != nil {
			t.Errorf("msgWriter failed to write: %s", msgwriter.err)
		}
		if !strings.Contains(buffer.String(), "--testboundary\r\n") {
			t.Errorf("expected boundary, got: %s", buffer.String())
		}
	})
	t.Run("msgWriter fails on S/MIME signing with broken rand.Reader", func(t *testing.T) {
		defaultRandReader := rand.Reader
		t.Cleanup(func() { rand.Reader = defaultRandReader })
		rand.Reader = &randReader{failon: 1}
		keypair, err := getDummyKeyPairTLS()
		if err != nil {
			t.Fatalf("failed to get dummy key material: %s", err)
		}

		buffer := bytes.NewBuffer(nil)
		msgwriter.writer = buffer
		message := testMessage(t)
		if err = message.SignWithTLSCertificate(keypair); err != nil {
			t.Fatalf("failed to initialize S/MIME signing: %s", err)
		}
		msgwriter.writeMsg(message)
		if msgwriter.err == nil {
			t.Errorf("msgWriter should have failed to write with broken rand.Reader")
		}
		expErr := "failed to read from rand.Reader: broken reader"
		if !strings.EqualFold(msgwriter.err.Error(), expErr) {
			t.Errorf("expected msgWriter to fail with: %s, got: %s", expErr, msgwriter.err)
		}
	})
}

func TestMsgWriter_writePreformattedGenHeader(t *testing.T) {
	t.Run("message with no preformatted headerset", func(t *testing.T) {
		buffer := bytes.NewBuffer(nil)
		msgwriter := &msgWriter{
			writer:  buffer,
			charset: CharsetUTF8,
			encoder: getEncoder(EncodingQP),
		}
		message := testMessage(t)
		message.SetGenHeaderPreformatted(HeaderContentID, "This is a content id")
		msgwriter.writeMsg(message)
		if !strings.Contains(buffer.String(), "Content-ID: This is a content id\r\n") {
			t.Errorf("expected preformatted header, got: %s", buffer.String())
		}
	})
}

func TestMsgWriter_addFiles(t *testing.T) {
	msgwriter := &msgWriter{
		charset: CharsetUTF8,
		encoder: getEncoder(EncodingQP),
	}
	tests := []struct {
		name     string
		filename string
		expect   string
	}{
		{"normal US-ASCII filename", "test.txt", "test.txt"},
		{"normal US-ASCII filename with space", "test file.txt", "test file.txt"},
		{"filename with new lines", "test\r\n.txt", "test__.txt"},
		{"filename with disallowed character:\x22", "test\x22.txt", "test_.txt"},
		{"filename with disallowed character:\x2f", "test\x2f.txt", "test_.txt"},
		{"filename with disallowed character:\x3a", "test\x3a.txt", "test_.txt"},
		{"filename with disallowed character:\x3c", "test\x3c.txt", "test_.txt"},
		{"filename with disallowed character:\x3e", "test\x3e.txt", "test_.txt"},
		{"filename with disallowed character:\x3f", "test\x3f.txt", "test_.txt"},
		{"filename with disallowed character:\x5c", "test\x5c.txt", "test_.txt"},
		{"filename with disallowed character:\x7c", "test\x7c.txt", "test_.txt"},
		{"filename with disallowed character:\x7f", "test\x7f.txt", "test_.txt"},
		{
			"japanese characters filename", "添付ファイル.txt",
			"=?UTF-8?q?=E6=B7=BB=E4=BB=98=E3=83=95=E3=82=A1=E3=82=A4=E3=83=AB.txt?=",
		},
		{
			"simplified chinese characters filename", "测试附件文件.txt",
			"=?UTF-8?q?=E6=B5=8B=E8=AF=95=E9=99=84=E4=BB=B6=E6=96=87=E4=BB=B6.txt?=",
		},
		{
			"cyrillic characters filename", "Тестовый прикрепленный файл.txt",
			"=?UTF-8?q?=D0=A2=D0=B5=D1=81=D1=82=D0=BE=D0=B2=D1=8B=D0=B9_=D0=BF=D1=80?= " +
				"=?UTF-8?q?=D0=B8=D0=BA=D1=80=D0=B5=D0=BF=D0=BB=D0=B5=D0=BD=D0=BD=D1=8B?= " +
				"=?UTF-8?q?=D0=B9_=D1=84=D0=B0=D0=B9=D0=BB.txt?=",
		},
	}
	for _, tt := range tests {
		t.Run("addFile with filename sanitization: "+tt.name, func(t *testing.T) {
			buffer := bytes.NewBuffer(nil)
			msgwriter.writer = buffer
			message := testMessage(t)
			message.AttachFile("testdata/attachment.txt", WithFileName(tt.filename))
			msgwriter.writeMsg(message)
			if msgwriter.err != nil {
				t.Errorf("msgWriter failed to write: %s", msgwriter.err)
			}

			var ctExpect string
			cdExpect := fmt.Sprintf(`Content-Disposition: attachment; filename="%s"`, tt.expect)
			switch runtime.GOOS {
			case "freebsd":
				ctExpect = fmt.Sprintf(`Content-Type: application/octet-stream; name="%s"`, tt.expect)
			default:
				ctExpect = fmt.Sprintf(`Content-Type: text/plain; charset=utf-8; name="%s"`, tt.expect)
			}
			if !strings.Contains(buffer.String(), ctExpect) {
				t.Errorf("expected content-type: %q, got: %q", ctExpect, buffer.String())
			}
			if !strings.Contains(buffer.String(), cdExpect) {
				t.Errorf("expected content-disposition: %q, got: %q", cdExpect, buffer.String())
			}
		})
	}
	t.Run("message with a single file attached", func(t *testing.T) {
		buffer := bytes.NewBuffer(nil)
		msgwriter.writer = buffer
		message := testMessage(t)
		message.AttachFile("testdata/attachment.txt")
		msgwriter.writeMsg(message)
		if msgwriter.err != nil {
			t.Errorf("msgWriter failed to write: %s", msgwriter.err)
		}
		switch runtime.GOOS {
		case "windows":
			if !strings.Contains(buffer.String(), "VGhpcyBpcyBhIHRlc3QgYXR0YWNobWVudA0K") {
				t.Errorf("attachment not found in mail message. Mail: %s", buffer.String())
			}
		default:
			if !strings.Contains(buffer.String(), "VGhpcyBpcyBhIHRlc3QgYXR0YWNobWVudAo=") {
				t.Errorf("attachment not found in mail message. Mail: %s", buffer.String())
			}
		}
		if !strings.Contains(buffer.String(), `Content-Disposition: attachment; filename="attachment.txt"`) {
			t.Errorf("Content-Disposition header not found for attachment. Mail: %s", buffer.String())
		}
		switch runtime.GOOS {
		case "freebsd":
			if !strings.Contains(buffer.String(), `Content-Type: application/octet-stream; name="attachment.txt"`) {
				t.Errorf("Content-Type header not found for attachment. Mail: %s", buffer.String())
			}
		default:
			if !strings.Contains(buffer.String(), `Content-Type: text/plain; charset=utf-8; name="attachment.txt"`) {
				t.Errorf("Content-Type header not found for attachment. Mail: %s", buffer.String())
			}
		}
	})
	t.Run("message with a single file attached no extension", func(t *testing.T) {
		buffer := bytes.NewBuffer(nil)
		msgwriter.writer = buffer
		message := testMessage(t)
		message.AttachFile("testdata/attachment")
		msgwriter.writeMsg(message)
		if msgwriter.err != nil {
			t.Errorf("msgWriter failed to write: %s", msgwriter.err)
		}
		switch runtime.GOOS {
		case "windows":
			if !strings.Contains(buffer.String(), "VGhpcyBpcyBhIHRlc3QgYXR0YWNobWVudA0K") {
				t.Errorf("attachment not found in mail message. Mail: %s", buffer.String())
			}
		default:
			if !strings.Contains(buffer.String(), "VGhpcyBpcyBhIHRlc3QgYXR0YWNobWVudAo=") {
				t.Errorf("attachment not found in mail message. Mail: %s", buffer.String())
			}
		}
		if !strings.Contains(buffer.String(), `Content-Disposition: attachment; filename="attachment"`) {
			t.Errorf("Content-Disposition header not found for attachment. Mail: %s", buffer.String())
		}
		if !strings.Contains(buffer.String(), `Content-Type: application/octet-stream; name="attachment"`) {
			t.Errorf("Content-Type header not found for attachment. Mail: %s", buffer.String())
		}
	})
	t.Run("message with a single file attached custom content-type", func(t *testing.T) {
		buffer := bytes.NewBuffer(nil)
		msgwriter.writer = buffer
		message := testMessage(t)
		message.AttachFile("testdata/attachment.txt", WithFileContentType(TypeAppOctetStream))
		msgwriter.writeMsg(message)
		if msgwriter.err != nil {
			t.Errorf("msgWriter failed to write: %s", msgwriter.err)
		}
		switch runtime.GOOS {
		case "windows":
			if !strings.Contains(buffer.String(), "VGhpcyBpcyBhIHRlc3QgYXR0YWNobWVudA0K") {
				t.Errorf("attachment not found in mail message. Mail: %s", buffer.String())
			}
		default:
			if !strings.Contains(buffer.String(), "VGhpcyBpcyBhIHRlc3QgYXR0YWNobWVudAo=") {
				t.Errorf("attachment not found in mail message. Mail: %s", buffer.String())
			}
		}
		if !strings.Contains(buffer.String(), `Content-Disposition: attachment; filename="attachment.txt"`) {
			t.Errorf("Content-Disposition header not found for attachment. Mail: %s", buffer.String())
		}
		if !strings.Contains(buffer.String(), `Content-Type: application/octet-stream; name="attachment.txt"`) {
			t.Errorf("Content-Type header not found for attachment. Mail: %s", buffer.String())
		}
	})
	t.Run("message with a single file attached custom transfer-encoding", func(t *testing.T) {
		buffer := bytes.NewBuffer(nil)
		msgwriter.writer = buffer
		message := testMessage(t)
		message.AttachFile("testdata/attachment.txt", WithFileEncoding(EncodingUSASCII))
		msgwriter.writeMsg(message)
		if msgwriter.err != nil {
			t.Errorf("msgWriter failed to write: %s", msgwriter.err)
		}
		if !strings.Contains(buffer.String(), "\r\n\r\nThis is a test attachment") {
			t.Errorf("attachment not found in mail message. Mail: %s", buffer.String())
		}
		if !strings.Contains(buffer.String(), `Content-Disposition: attachment; filename="attachment.txt"`) {
			t.Errorf("Content-Disposition header not found for attachment. Mail: %s", buffer.String())
		}
		switch runtime.GOOS {
		case "freebsd":
			if !strings.Contains(buffer.String(), `Content-Type: application/octet-stream; name="attachment.txt"`) {
				t.Errorf("Content-Type header not found for attachment. Mail: %s", buffer.String())
			}
		default:
			if !strings.Contains(buffer.String(), `Content-Type: text/plain; charset=utf-8; name="attachment.txt"`) {
				t.Errorf("Content-Type header not found for attachment. Mail: %s", buffer.String())
			}
		}
		if !strings.Contains(buffer.String(), `Content-Transfer-Encoding: 7bit`) {
			t.Errorf("Content-Transfer-Encoding header not found for attachment. Mail: %s", buffer.String())
		}
	})
	t.Run("message with a single file attached custom description", func(t *testing.T) {
		buffer := bytes.NewBuffer(nil)
		msgwriter.writer = buffer
		message := testMessage(t)
		message.AttachFile("testdata/attachment.txt", WithFileDescription("Testdescription"))
		msgwriter.writeMsg(message)
		if msgwriter.err != nil {
			t.Errorf("msgWriter failed to write: %s", msgwriter.err)
		}
		switch runtime.GOOS {
		case "windows":
			if !strings.Contains(buffer.String(), "VGhpcyBpcyBhIHRlc3QgYXR0YWNobWVudA0K") {
				t.Errorf("attachment not found in mail message. Mail: %s", buffer.String())
			}
		default:
			if !strings.Contains(buffer.String(), "VGhpcyBpcyBhIHRlc3QgYXR0YWNobWVudAo=") {
				t.Errorf("attachment not found in mail message. Mail: %s", buffer.String())
			}
		}
		if !strings.Contains(buffer.String(), `Content-Disposition: attachment; filename="attachment.txt"`) {
			t.Errorf("Content-Disposition header not found for attachment. Mail: %s", buffer.String())
		}
		switch runtime.GOOS {
		case "freebsd":
			if !strings.Contains(buffer.String(), `Content-Type: application/octet-stream; name="attachment.txt"`) {
				t.Errorf("Content-Type header not found for attachment. Mail: %s", buffer.String())
			}
		default:
			if !strings.Contains(buffer.String(), `Content-Type: text/plain; charset=utf-8; name="attachment.txt"`) {
				t.Errorf("Content-Type header not found for attachment. Mail: %s", buffer.String())
			}
		}
		if !strings.Contains(buffer.String(), `Content-Transfer-Encoding: base64`) {
			t.Errorf("Content-Transfer-Encoding header not found for attachment. Mail: %s", buffer.String())
		}
		if !strings.Contains(buffer.String(), `Content-Description: Testdescription`) {
			t.Errorf("Content-Description header not found for attachment. Mail: %s", buffer.String())
		}
	})
	t.Run("message with attachment but no body part", func(t *testing.T) {
		buffer := bytes.NewBuffer(nil)
		msgwriter.writer = buffer
		message := testMessage(t)
		message.parts = nil
		message.AttachFile("testdata/attachment.txt")
		msgwriter.writeMsg(message)
		if msgwriter.err != nil {
			t.Errorf("msgWriter failed to write: %s", msgwriter.err)
		}
		switch runtime.GOOS {
		case "windows":
			if !strings.Contains(buffer.String(), "VGhpcyBpcyBhIHRlc3QgYXR0YWNobWVudA0K") {
				t.Errorf("attachment not found in mail message. Mail: %s", buffer.String())
			}
		default:
			if !strings.Contains(buffer.String(), "VGhpcyBpcyBhIHRlc3QgYXR0YWNobWVudAo=") {
				t.Errorf("attachment not found in mail message. Mail: %s", buffer.String())
			}
		}
		if !strings.Contains(buffer.String(), `Content-Disposition: attachment; filename="attachment.txt"`) {
			t.Errorf("Content-Disposition header not found for attachment. Mail: %s", buffer.String())
		}
		switch runtime.GOOS {
		case "freebsd":
			if !strings.Contains(buffer.String(), `Content-Type: application/octet-stream; name="attachment.txt"`) {
				t.Errorf("Content-Type header not found for attachment. Mail: %s", buffer.String())
			}
		default:
			if !strings.Contains(buffer.String(), `Content-Type: text/plain; charset=utf-8; name="attachment.txt"`) {
				t.Errorf("Content-Type header not found for attachment. Mail: %s", buffer.String())
			}
		}
		if !strings.Contains(buffer.String(), `Content-Transfer-Encoding: base64`) {
			t.Errorf("Content-Transfer-Encoding header not found for attachment. Mail: %s", buffer.String())
		}
	})
}

func TestMsgWriter_writePart(t *testing.T) {
	msgwriter := &msgWriter{
		charset: CharsetUTF8,
		encoder: getEncoder(EncodingQP),
	}
	t.Run("message with no part charset should use default message charset", func(t *testing.T) {
		buffer := bytes.NewBuffer(nil)
		msgwriter.writer = buffer
		message := testMessage(t, WithCharset(CharsetUTF7))
		message.AddAlternativeString(TypeTextPlain, "thisisatest")
		message.parts[1].charset = ""
		msgwriter.writeMsg(message)
		if msgwriter.err != nil {
			t.Errorf("msgWriter failed to write: %s", msgwriter.err)
		}
		if !strings.Contains(buffer.String(), "ontent-Type: text/plain; charset=UTF-7\r\n\r\nTestmail") {
			t.Errorf("part not found in mail message. Mail: %s", buffer.String())
		}
		if !strings.Contains(buffer.String(), "ontent-Type: text/plain; charset=UTF-7\r\n\r\nthisisatest") {
			t.Errorf("part not found in mail message. Mail: %s", buffer.String())
		}
	})
	t.Run("message with parts that have a description", func(t *testing.T) {
		buffer := bytes.NewBuffer(nil)
		msgwriter.writer = buffer
		message := testMessage(t)
		message.AddAlternativeString(TypeTextPlain, "thisisatest")
		message.parts[1].description = "thisisadescription"
		msgwriter.writeMsg(message)
		if msgwriter.err != nil {
			t.Errorf("msgWriter failed to write: %s", msgwriter.err)
		}
		if !strings.Contains(buffer.String(), "Content-Description: thisisadescription") {
			t.Errorf("part description not found in mail message. Mail: %s", buffer.String())
		}
	})
}

func TestMsgWriter_writeString(t *testing.T) {
	msgwriter := &msgWriter{
		charset: CharsetUTF8,
		encoder: getEncoder(EncodingQP),
	}
	t.Run("writeString succeeds", func(t *testing.T) {
		buffer := bytes.NewBuffer(nil)
		msgwriter.writer = buffer
		msgwriter.writeString("thisisatest")
		if !strings.EqualFold(buffer.String(), "thisisatest") {
			t.Errorf("writeString failed, expected: thisisatest got: %s", buffer.String())
		}
	})
	t.Run("writeString fails", func(t *testing.T) {
		msgwriter.writer = failReadWriteSeekCloser{}
		msgwriter.writeString("thisisatest")
		if msgwriter.err == nil {
			t.Errorf("writeString succeeded, expected error")
		}
	})
	t.Run("writeString on errored writer should return", func(t *testing.T) {
		buffer := bytes.NewBuffer(nil)
		msgwriter.writer = buffer
		msgwriter.err = errors.New("intentional error")
		msgwriter.writeString("thisisatest")
		if !strings.EqualFold(buffer.String(), "") {
			t.Errorf("writeString succeeded, expected: empty string, got: %s", buffer.String())
		}
	})
}

func TestMsgWriter_writeHeader(t *testing.T) {
	msgwriter := &msgWriter{
		charset: CharsetUTF8,
		encoder: getEncoder(EncodingQP),
	}
	t.Run("writeHeader with single value", func(t *testing.T) {
		buffer := bytes.NewBuffer(nil)
		msgwriter.writer = buffer
		msgwriter.writeHeader(HeaderMessageID, "this.is.a.test")
		if !strings.EqualFold(buffer.String(), "Message-ID: this.is.a.test\r\n") {
			t.Errorf("writeHeader failed, expected: %s, got: %s", "Message-ID: this.is.a.test",
				buffer.String())
		}
	})
	t.Run("writeHeader with multiple values", func(t *testing.T) {
		buffer := bytes.NewBuffer(nil)
		msgwriter.writer = buffer
		msgwriter.writeHeader(HeaderMessageID, "this.is.a.test", "this.as.well")
		if !strings.EqualFold(buffer.String(), "Message-ID: this.is.a.test, this.as.well\r\n") {
			t.Errorf("writeHeader failed, expected: %s, got: %s", "Message-ID: this.is.a.test, this.as.well",
				buffer.String())
		}
	})
	t.Run("writeHeader with no values", func(t *testing.T) {
		buffer := bytes.NewBuffer(nil)
		msgwriter.writer = buffer
		msgwriter.writeHeader(HeaderMessageID)
		// While technically it is permitted to have empty headers, it's recommend to omit them if
		// no value is present. We follow this recommendation.
		if !strings.EqualFold(buffer.String(), "") {
			t.Errorf("writeHeader failed, expected: %s, got: %s", "", buffer.String())
		}
	})
	t.Run("writeHeader with very long value", func(t *testing.T) {
		buffer := bytes.NewBuffer(nil)
		msgwriter.writer = buffer
		msgwriter.writeHeader(HeaderMessageID, strings.Repeat("a", MaxHeaderLength-13), "next-row")
		want := "Message-ID:\r\n " + strings.Repeat("a", MaxHeaderLength-13) + ",\r\n next-row\r\n"
		if !strings.EqualFold(buffer.String(), want) {
			t.Errorf("writeHeader failed, expected: %s, got: %s", want, buffer.String())
		}
	})
}

func TestMsgWriter_writeBody(t *testing.T) {
	t.Log("We only cover some edge-cases here, most of the functionality is tested already very thoroughly.")

	msgwriter := &msgWriter{
		charset: CharsetUTF8,
		encoder: getEncoder(EncodingQP),
	}
	t.Run("writeBody on NoEncoding", func(t *testing.T) {
		buffer := bytes.NewBuffer(nil)
		msgwriter.writer = buffer
		message := testMessage(t)
		msgwriter.writeBody(message.parts[0].writeFunc, NoEncoding)
		if msgwriter.err != nil {
			t.Errorf("writeBody failed to write: %s", msgwriter.err)
		}
	})
	t.Run("writeBody on NoEncoding fails on write", func(t *testing.T) {
		msgwriter.writer = failReadWriteSeekCloser{}
		message := testMessage(t)
		msgwriter.writeBody(message.parts[0].writeFunc, NoEncoding)
		if msgwriter.err == nil {
			t.Errorf("writeBody succeeded, expected error")
		}
		if !strings.EqualFold(msgwriter.err.Error(), "bodyWriter io.Copy: intentional write failure") {
			t.Errorf("expected error: bodyWriter io.Copy: intentional write failure, got: %s", msgwriter.err)
		}
	})
	t.Run("writeBody on NoEncoding fails on writeFunc", func(t *testing.T) {
		buffer := bytes.NewBuffer(nil)
		msgwriter.writer = buffer
		writeFunc := func(io.Writer) (int64, error) {
			return 0, errors.New("intentional write failure")
		}
		msgwriter.writeBody(writeFunc, NoEncoding)
		if msgwriter.err == nil {
			t.Errorf("writeBody succeeded, expected error")
		}
		if !strings.EqualFold(msgwriter.err.Error(), "bodyWriter function: intentional write failure") {
			t.Errorf("expected error: bodyWriter function: intentional write failure, got: %s", msgwriter.err)
		}
	})
	t.Run("writeBody Quoted-Printable fails on write", func(t *testing.T) {
		msgwriter.writer = failReadWriteSeekCloser{}
		message := testMessage(t)
		msgwriter.writeBody(message.parts[0].writeFunc, EncodingQP)
		if msgwriter.err == nil {
			t.Errorf("writeBody succeeded, expected error")
		}
		if !strings.EqualFold(msgwriter.err.Error(), "bodyWriter function: intentional write failure") {
			t.Errorf("expected error: bodyWriter function: intentional write failure, got: %s", msgwriter.err)
		}
	})
	t.Run("writeBody Quoted-Printable fails on writeFunc", func(t *testing.T) {
		buffer := bytes.NewBuffer(nil)
		msgwriter.writer = buffer
		writeFunc := func(io.Writer) (int64, error) {
			return 0, errors.New("intentional write failure")
		}
		msgwriter.writeBody(writeFunc, EncodingQP)
		if msgwriter.err == nil {
			t.Errorf("writeBody succeeded, expected error")
		}
		if !strings.EqualFold(msgwriter.err.Error(), "bodyWriter function: intentional write failure") {
			t.Errorf("expected error: bodyWriter function: intentional write failure, got: %s", msgwriter.err)
		}
	})
}

func TestMsgWriter_sanitizeFilename(t *testing.T) {
	tests := []struct {
		given string
		want  string
	}{
		{"test.txt", "test.txt"},
		{"test file.txt", "test file.txt"},
		{"test\\ file.txt", "test_ file.txt"},
		{`"test" file.txt`, "_test_ file.txt"},
		{`test	file	.txt`, "test_file_.txt"},
		{"test\r\nfile.txt", "test__file.txt"},
		{"test\x22file.txt", "test_file.txt"},
		{"test\x2ffile.txt", "test_file.txt"},
		{"test\x3afile.txt", "test_file.txt"},
		{"test\x3cfile.txt", "test_file.txt"},
		{"test\x3efile.txt", "test_file.txt"},
		{"test\x3ffile.txt", "test_file.txt"},
		{"test\x5cfile.txt", "test_file.txt"},
		{"test\x7cfile.txt", "test_file.txt"},
		{"test\x7ffile.txt", "test_file.txt"},
	}
	for _, tt := range tests {
		t.Run(tt.given+"=>"+tt.want, func(t *testing.T) {
			if got := sanitizeFilename(tt.given); got != tt.want {
				t.Errorf("sanitizeFilename failed, expected: %q, got: %q", tt.want, got)
			}
		})
	}
}