Hey fellow bubblers, I just released an update that allows you to get the Message ID from the workflow actions.
Single email returns a single text and batch email returns a list of texts.
Get them with the “Result of step X” mechanism!
It is optional to use that feature, useful for advanced logging.
You can track news on the plugin tab itself (when the plugin says “upgrade available”) and also this dedicated thread here, check it out 
: Richer Postmark emails plugin 
[Update of 16 march 2021]