After enabling the error report log I'm now seeing this error in the logs:
[ ID 165] Error Sending Message Your Subscription Expires Soon to USEREMAILADDRESSREMOVEDMANUALLY | SMTP Error: data not accepted.SMTP server error: DATA END command failed Detail: STOREDRV.Submission.Exception:SendAsDeniedException.MapiExceptionSendAsDenied; Failed to process message due to a permanent exception with message Cannot submit message. 0.35250:CF050000, 1.36674:0A000000, 1.61250:00000000, 1.45378:02000000, 1.44866:45380000, 1.36674:0E000000, 1.61250:00000000, 1.45378:4A380000, 1.44866:42010000, 16.55847😃C220000, 17.43559:0000000004020000000000000000000000000000, 20.52176:140FBE8A1300000000000000, 20.50032:140FBE8A831770200A00D180, 0.35180:140FBE8A, 255.23226:ED05 SMTP code: 554 Additional SMTP info: 5.2.0 SMTP Error: data not accepted. => Next try in 60 minutes
So Acymailing IS reporting an error; however, it is happening over 2 hours after the cron has run to add the email to the user and I'm not at all familiar with these error notices to know why this would be causing a problem, while sending the exact same email manually from within AcyMailing works without a problem?