During a recent Exchange 2010 -> 2013 SP1 migration, I had a couple of mailboxes whose security settings were corrupted. Thankfully, I was able to Export the mailbox content, delete the old mailboxes, create new mailboxes with default security settings, and import the data back onto the new mailboxes. Unfortunately, doing so has left me with what seems to be a client-side cache problem. When any user who has sent the new mailboxes mail in the past sends a message, they receive a "5.1.1 smtp;550 5.1.1 resolver.adr.recipnotfound; not found " reply, even thought the SMTP addresses are exactly the same as the old accounts. Clearing the cache on the sending client seems to do the trick. This is very painful, as not everyone user is cut out to do this. Is there any solution to this problem?