On Unity 8.x I had a monitor on the UnityMTA folder as an easy way of knowing when messages weren't being passed to Exchange.
I'm looking for an alert to notify me on CUC v9.1(2) (SIB) when messages cannot sync with a user's exchange mailbox. I'd like to know of any problems, even just for a single user, before the user knows. Any suggestions?
Unfortunately, I don't think there is any intuitive or simple way to get what you want here. There are Single Inbox Traces via the Connection Mailbox Sync service but you'd then need some sort of automated way to pull/parse that data and I think that would be rather cumbersome.