Add Stalwart SMTP to Communication - Email - MTAs (#3634)

Co-authored-by: nodiscc <nodiscc@gmail.com>

README.md

@@ -434,6 +434,7 @@ Simple deployment of [E-mail](https://en.wikipedia.org/wiki/Email) servers, e.g.
 - [Postfix](http://www.postfix.org/) - Fast, easy to administer, and secure Sendmail replacement. `IPL-1.0` `C`
 - [Sendmail](https://www.proofpoint.com/us/products/email-protection/open-source-email-solution) - Message transfer agent (MTA). `Sendmail` `C`
 - [Slimta](https://www.slimta.org) - Mail Transfer Library built on Python. ([Source Code](https://github.com/slimta/python-slimta)) `MIT` `Python`
+- [Stalwart SMTP](https://stalw.art/smtp) - Modern SMTP server designed with a focus on security, speed, and extensive configurability. ([Source Code](https://github.com/stalwartlabs/smtp-server)) `AGPL-3.0` `Rust`
 - [vSMTP](https://viridit.com/vsmtp-mail-transfer-agent/) - Next-gen MTA. Secured, Faster and Greener. ([Source Code](https://github.com/viridIT/vSMTP)) `GPL-3.0` `Rust`