Linux Babe published a tutorial about setting up SMTP and IMAP Proxy with HAProxy.

Set Up SMTP and IMAP Proxy with HAProxy (Debian, Ubuntu, CentOS)

This tutorial is going to show you how to set up SMTP and IMAP proxy for your mail server with HAProxy. You can run a VPS (Virtual Private Server) at a data center and use it as a proxy for your mail server.

Some folks run email servers at home, but may have the following problems: Port 25 is blocked, They don’t have a static IP address, They can’t create PTR record. If port 25 is blocked, you can’t send emails directly to recipients. And if you don’t have static IP address or PTR record, your emails are likely to be rejected or land into the spam folder.

Set Up SMTP and IMAP Proxy with HAProxy (Debian, Ubuntu, CentOS)