Stop any and all use of Mail.app's database (Envelope Index). In some cases
this was causing corruption in Envelope Index. You can check
for this in Terminal (while not running Mail) with:
This can take a long time if you have a large number of mail messages.
If you get anything besides "ok", then you should rebuild your Envelope
Index file and your RelatedMail indices by following these
instructions at the end of the Known Issues section.
sqlite3 ~/Library/Mail/Envelope\ Index "pragma