If you see an event a mailbox approaching the storage limit, but the mailbox ID is just a GUID, here’s how you determine whose mailbox it is.
Open Exchange Management Shell and run the following command
Get-Mailbox <Long GUID Here>
The output will give you the actual user.
I recently completed an Exchange 2010 to 2016 migration. The last few days I’ve been getting word of quite a few users receiving NDRs. The NDR error was QUEUE Expired. I went straight to the queue and saw there were currently 12 messages there all with a last error of “A storage transient failure has occurred during content conversion”. I did eventually find a solution, and I normally don’t repost items I find in my day to day tech support life, but there wasn’t a ton of results for this error, and most weren’t relevant. But then I found the what I was looking for. This link had some Exchange Management Shell commands that allowed all the messages in the queue to send instantly
Update [3/31/16]: This issue is not resolved for me. With the setting set to null or false, I get some messages stuck in the submission queue. If the TNEFEnabled is set to true, all mail sends fine, but certain recipients receive attachments as winmail.dat. So I’m still working on this one.
Update 2 [4/19/16]: I installed Cumulative Update 1 for Exchange 2016 almost a week ago and the issue appears to be resolved. I have not had a single message stuck in the Submission Queue.