I have my additional email address setup with it own inbox. The setup has been the same for 2 years, but recently when I delete emails and later click get mail, some of them download from the server again. It doesn't happen to every email I delete and never with the primary email address inbox. I thought maybe the trash folder was too full, so I emptied it, but the problem still occurs. I've even tried deleting emails by pressing shift/delete so they don't go into the trash folder, but this doesn't matter either. I haven't setup Thunderbird to delete messages from the server once they are downloaded as I sometimes access emails from my phone and laptop when away from my desktop.