Due to the fact that the mail sub board is not listed in the standard sub_list array, Mail> is not hit by a new message scan. There should be some code thrown into the new message scan that always checks the Mail> at the outset of a scan (or the first time a new room is looked for), in order to make sure that new mail is received as soon as possible.
It would be really nice if there were a way to customize the notification for new emails (if that's not disabled by the different Synchronet bits in the user configuration) to make something that looks a little more like vDoc. That's not important at this point, though.