Preserve the Correct Source IP When Forwarding Mailcow TCP Traffic

Thu, 07 May 2026 00:10:22 +0000

When Mailcow runs on a separate machine, forwarding its TCP traffic through HAProxy can leave the logs showing the forwarding layer instead of the connecting client. That defeats the point of checking the mail logs when the original source IP matters.

The service still receives traffic, so the forwarding path can look healthy at first. The problem becomes visible when investigating delivery, abuse, rate limits, or client behaviour and every connection appears to originate from the proxy instead of the real sender.

Mailcow Outbound Mail Fails When It Uses the Wrong Public IP

Thu, 07 May 2026 00:10:21 +0000

Mailcow was sending from the public IP of the machine where it was installed. That was the wrong address for this domain: the SPF record expected mail to leave through a different machine.

The server could send mail, so the problem did not first look like networking. The important detail appeared on the receiving side. The message was evaluated against the public IP that actually delivered it, not the public IP that the domain owner expected it to use.

Email - Tag - Dimas Maulana

Preserve the Correct Source IP When Forwarding Mailcow TCP Traffic

Mailcow Outbound Mail Fails When It Uses the Wrong Public IP