mail minus
I just upgraded my iMac to Sierra. Strange thing happened: a minus (-1) appeared on my Mail icon. Any reason for that?
iMac, Mac OS X (10.5.8)
Try rebuilding the Mailbox database according to the following: Mail for Mac: Rebuild mailboxes.
It will take a few moments to complete, depending on the total number of messages in those Mailboxes. Mail's operation will also be slow until it finishes.
It would be prudent to back up your Mac prior to doing that, but if rebuilding a Mailbox causes any email messages to be lost they were probably not recoverable anyway. I recommend backing up in this particular circumstance only in an abundance of caution, but it's something everyone ought to be doing.