Add `vSMTP` to `Communication - Email - Mail Transfer Agents` (#3152)

Co-authored-by: Mathieu-Lala <m.lala@viridit.com>

README.md

@@ -404,6 +404,7 @@ MTAs / SMTP servers
 - [Qmail](https://cr.yp.to/qmail.html) - Secure Sendmail replacement. ([Source Code](https://sources.debian.net/src/netqmail/1.06-5/)) `CC0-1.0` `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`
+- [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`
 
 
 ### Communication - Email - Mailing Lists and Newsletters