New mail notifications do not always work reliably.
Sometimes, after a copy operation (of some size), mails are not shown after selecting the destination folder. Restarting tb shows that they were actually copied, so this must be a problem of notification.
I suspect it has to do with the result of the select arriving before the copy has finished.
(from redmine: created on 2014-02-24, closed on 2014-04-03, relates #5165 (closed), duplicates #5187 (closed))