What is pm.mtasv.net?

pm.mtasv.net is the hostname used by Postmark for their bounce tracking. It shows up in the headers of an email as

Return-Path: pm_bounces@pm.mtasv.net

What is a "Return Path"?

When an email server receives an email that it cannot handle for whatever reason, it needs to be report that back to the sender. The Return-Path email address is where the email server sends that error report.

The Return-Path address is also commonly known as a bounce address because it is used for "bounces"; ie. emails that cannot be delivered and is bounced back to the sender.

Having a custom Return-Path address that matches your domain name is important in order to achieve SPF-alignment.

Guides

See also