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Postmark have recently opened up the ability to use their services for 'broadcast' messages.
I can see that in Sendy I can setup SMTP as a method for sending msgs.
Here is what postmark say about setting up SMTP for them:
To send with Broadcast Message Streams via SMTP, it requires the ability to add additional Headers with your SMTP client. The X-PM-Message-Stream header is your Broadcast Message Stream ID.
Ports: 25, 2525, or 587
Username: server token
Password: server token
Authentication: Plain text, CRAM-MD5, or TLS
Header: X-PM-Message-Stream: broadcasts
As you can see it requires the ability to add additional Headers with the SMTP client (sendy).
How can I configure this in Sendy to do this?