Add github.com/mjl-/mox, a modern email server (#4919)

* Add github.com/mjl-/mox, a modern email server

Mox is a modern full-featured open source secure mail server for
low-maintenance self-hosted email. It implements IMAP, SMTP, SPF, DKIM, DMARC,
TLSRPT, MTA-STS and more.

* chore: grammar correction

---------

Co-authored-by: Yassine Benaid <yassinebenaide3@gmail.com>

README.md

@@ -1058,6 +1058,7 @@ _Libraries and tools that implement email creation and sending._
 - [MailHog](https://github.com/mailhog/MailHog) - Email and SMTP testing with web and API interface.
 - [Mailpit](https://github.com/axllent/mailpit) - Email and SMTP testing tool for developers.
 - [mailx](https://github.com/valord577/mailx) - Mailx is a library that makes it easier to send email via SMTP. It is an enhancement of the golang standard library `net/smtp`.
+- [mox](https://github.com/mjl-/mox) - Modern full-featured secure mail server for low-maintenance, self-hosted email.
 - [SendGrid](https://github.com/sendgrid/sendgrid-go) - SendGrid's Go library for sending email.
 - [smtp](https://github.com/mailhog/smtp) - SMTP server protocol state machine.
 - [smtpmock](https://github.com/mocktools/go-smtp-mock) - Lightweight configurable multithreaded fake SMTP server. Mimic any SMTP behaviour for your test environment.