Add note regarding missing PTR/RDNS record

Author: jaapmarcus

CHANGELOG.md

@@ -6,6 +6,7 @@ All notable changes to this project will be documented in this file.
 ### Breaking changes ###
 - **NOTE:** Changes have been made on how phpmyadmin/phppgadmin config are included in apache2 config. To restore to the old behaviour add `IncludeOptional conf.d/*.inc` below `IncludeOptional conf.d/*.conf` in /etc/apache2/apache2.conf and restart your server. 
 - **NOTE:** Hestia packages for arm64 has been added to atp.hestiacp.com please use the normal install instructions instead! For current ARM installs to enable auto update remove the `#` in /etc/apt/sources.list.d/hestia.list `# deb https://apt.hestiacp.com/ focal main` becomes `deb https://apt.hestiacp.com/ focal main` and then run `apt update && apt upgrade -y` 
+- **NOTE:** Make sure your server / VPS has a valid PTR record or otherwise you will not be able to send any mail!
 
 ### Features