Bartłomiej Piotrowski13000 messages in one folder is definitely too many for Thunderbird. Unread mails get displayed, but the list of messages gets unbearably slow.
Of course I could switch to mutt, but I do prefer somewhat sane defaults without spending too much on tweaking and learning every single thing.
Last time I tried it, I accidentally removed all messages from my inbox.
ONE SIMPLE TRICK TO ACHIEVE INBOX ZERO