Sometimes there are -1 new and unread mails mentioned in the tooltip. The icon shows then the status "new mail available".
Please see attached screen shot.
I can imagine that it has to do with the delayed but automatically unmarking of new mails.
Configuration > Preferences > Display/Summaries > Message list > Mark message as read >
when selected after (3) seconds
Created attachment 1157 [details]
-1 new and unread mails
Is there a way to reproduce this?
Honestly, I don't know how to reproduce for sure. I guess that it has something to do with the delay feature to unmark new mails (see second part of comment #1).
Doing a resync with the IMAP4 server fixes the counter. So I reduce Severity to minor cause of available (but not nice usability) work around.