YAM supports multiple users. Each of these users must have a real name to enable YAM to distinguish between these users. This name must be set in YAM's user management.
The problem with the automatic mail exchange on startup arises if a user doesn't have a real name. In this case YAM 2.6 cannot detect that upon a (re)start the user has changed and hence it will skip the automatic mail exchange completely. However this can only happen if you never ever opened YAM's user management to give the user a real name.
The solution is very simple: open YAM's user management window, check all user names to be correct and close the window again. After that the automatic mail exchange upon start will work again.
The Online-FAQ of www.yam.ch