What are the SMTP details / credentials I should be using?
SMTP endpoint: smtp.postmarkapp.com
Port 25, Port 2525 or Port 587
Username and Password: Your full Server API token which can be found in the Server > Credentials tab in your Postmark account. This is unique for each server you create in Postmark.
If you are using Postfix to send through Postmark you can follow our Instructions for getting set up in our guide Can I configure Postfix to send through Postmark?