Nearly all of the Exchange issues I deal with right now really come from O365.
However I was asked about Exchange Mail Submission server that was throwing Event 1009, and effectively stopping mail flow. What he found via forum (not ML) searching was 'a back pressure issue, but the Event Log wasn't logging in back pressure messages'
While it's been four years since directly touching a Exchange system, the term 'back pressure' is not one I have heard before.
However, looking up Exchange Event 1009 give me a few options including limited disk space.
Has anyone come across the term 'Back Pressure' with Exchange in the past?
Yes, it is when the server is running out of space or resources for delivery of incoming emails