You may notice in your DMARC report that emails sent through Postmark are showing as failing SPF, even though you have correctly including Postmark in your SPF record. As with most email service providers, Postmark uses a custom domain to collect bounces through the "Return-Path" header in emails. This address resides at the domain pm.mtasv.net.
With DMARC, the Return-Path and From address must match for SPF alignment. This means that ESPs will fail the SPF DMARC alignment unless a custom return-path is used. Don’t worry though, DMARC only requires either SPF or DKIM to be aligned. If you have DKIM authentication in place, these emails that fail SPF will still pass DMARC.
Some ESPs get around this by using a Sender header, but we never liked that option due to the “on behalf of” message that can show up in email clients. In addition, we like our customers to build a reputation on their own domains by using custom DKIM records in their DNS.
To fully support DMARC when sending emails from Postmark, you can add a custom Return-Path domain for your own domain. This will allow the Return-Path to match the From address, resulting in a passing SPF DMARC alignment for your emails.
Check out our support article on adding a custom Return-Path domain here.