I'm connecting to an Exchange 2010 server and storing my sent messages locally.
Once I've replied to a messages in my Inbox, the usual arrow appears to the left of the message. But when I click on the arrow, roughly 50% of the time Mail fails: nothing comes up at all when I click. (A message I sent minutes earlier/later can work just fine.)
The replies WERE sent using Apple Mail (not my iPhone) and it ARE in my sent mail folder. I have re-built my Spotlight index.