Added Postal Email solution

Self hosted mail server for incoming/outgoing

README.md

@@ -265,6 +265,7 @@ _simple deployment of a mail server, e.g. for inexperienced or impatient admins.
   * [Mailcow](https://mailcow.email/) - Mail server suite based on Dovecot, Postfix and other open source software, that provides a modern Web UI for administration. ([Source Code](https://github.com/andryyy/mailcow)) `GPLv2` `PHP`
   * [Mail-in-a-Box](https://mailinabox.email/) - Turns any Ubuntu server into a fully functional mail server with one command. ([Source Code](https://github.com/mail-in-a-box/mailinabox)) `CC0` `Shell`
   * [Modoboa](http://modoboa.org/en/) - Modoboa is a mail hosting and management platform including a modern and simplified Web User Interface. ([Source Code](https://github.com/tonioo/modoboa)) `MIT` `Python`
+  * [Postal](https://github.com/atech/postal) - Postal is a complete and full featured mail server for use by websites and web servers. `MIT` `Ruby`
 
 #### Mail Transfer Agents