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.

Spf - Tag - Dimas Maulana

Mailcow Outbound Mail Fails When It Uses the Wrong Public IP