PostmarkApp is capable of sending DMARC compliant email because they support custom SPF and DKIM configurations that achieve alignment. This source is capable of sending a number of different message types, ranging from transactional messages to marketing. * PostmarkApp is often used as a back end SMTP service, which means that its primary users will be Software Development teams, even though they may be sending on behalf of different departments. This source may also be in use by another 3rd party sending messages on your behalf, making it a bit challenging to identify who the true owner of this resource is.
The link just below is a resource on how to configure DKIM. You will complete this configuration by (1) following the sources instructions and (2) adding the required entries into your DNS. https://postmarkapp.com/support/article/1091-how-do-i-set-up-dkim-for-postmark
The link just below is a resource on how to configure SPF. You will complete this configuration by (1) following the sources instructions and (2) adding the required entries into your DNS. https://postmarkapp.com/videos/dkim-and-custom-return-path-authentication