I might be wrong but I think it's new since we updated to this latest build.
We now have a lot of system messages stuck in spool with many retries.
For, example, system messages notifying users that their mailbox is full are not delivered because the mailbox is ... full.
Aren't the system messages supposed to be delivered even if the mailbox is full ?
I think it was until now...
EDIT: These are "events" trigerred messages, not system messages, but still they were previously delivered even if the target mailbox was full. Not anymore.