Have confirmed the mailbox has plenty of space (only 8% usage of a large mailbox and client cleaned out a bunch as a preventative measure), mail is slowly flowing from OE but many are still sitting in the Queue for a day or two and receiving that error message when we attempt to force send.

ST Support team, when you've settled after your move could you toss a couple suggestions if you have them, doesn't look like many others have experienced this issue.