Simple Mail Transfer Protocol (SMTP) Setup
A howto for proxying this protocol. We recommend using a Tunnel for this protocol (GRE/IP-in-IP or IPSec).
Just a standard TCP port. No custom configuration necessary. For the most security ensure your hostname does not leak information about your backend. Ensure your mail server does not reject based on SPF records, connections will originate from the proxy server.
We recommend using a tunnel for SMTP servers to ensure that SPF record checking and spam filtering (and reporting) works as designed.