As we are managing the migration from Exchange to the SM we have on occasion had to do multiple migrations on large accounts because of failures that occur during the migration leading to lost messages. I know there is an option to wipe the mailbox but that isn't a good option because new mail is arriving and can't be lost. How do others handle this problem, or am I the only on who is seeing occasional migration failures? 

I worked around this by creating a temp account, migrate the exchange data until I get a good copy, then do a migration from the temp SM account to the destination account after doing a mass delete of everything that originally came from the exchange server but was incomplete. Yeah, it was ugly, took a lot of time, but in the end it worked. I don't recommend it without a good bourbon by your side.

Idea: What if SM added an option to delete all mail older than xx/xx/xx date rather than the whole account.