When I check and delete lots of email at once I often (but not always, apparently) experience a problem: the mail will still show in inbox until I empty the trash folder. This was a bit confusing at first and more so for other less experienced users but is not a critical bug as there is an easy workaround (empty trash). Also, it sometimes happens that after deleting mail I see in the refreshed inbox not the messages I'd expect (those I didn't delete) but instead I see some of the deleted messages or a mix and match of them. It seems that squirrelmail will display those that were in the first positions before deletion, no matter their current status. In this case it is sufficient to click on inbox again to receive the current, correct, inbox contents on screen.
Please, report this upstream, not a Gentoo-specific issue. http://sourceforge.net/tracker/?group_id=311&atid=100311