I still cannot find a solution to this weird problem, any ideas welcomed.
This came up after upgrading to Mavericks. Using Mail.app, multiple mail accounts. It only happens with my sole Exchange account.
Upon opening mail, the display is correct for several seconds. Then the screen seems to 'refresh', and the sender's name in the middle pane is replaced by the name of the first recipient. If I switch to a different mail account, then back to the Exchange account, the display is again correct for a few seconds. This phenonenom also affects spotlight search by sender in an irritating way, since when the display changes, the result of the search clears.
Have tried: rebuilding mail index, both from within mail and brute-force (deleting the files); deleting the Exchange account from within mail, then adding it back in.