Is this a pop or imap mail account? First make sure your Account Settings are doing what you want. For example: you may also want to make sure emails are deleted off the server when you delete them in Thunderbird. Suggest this setting: * Right click on pop mail account in Folder Pane and select 'Settings' * select: 'Server Settings' * select: 'Leave message on server' * Select: 'Until I delete them' * click on OK What could cause a load of previously deleted emails to reappear? Two things come to mind either they were resurrected or downloaded again. I'm guessing this is pop, but would need you to confirm. If pop: '''If those emails were 'resurrected' '''- not downloaded - then I would be a bit concerned if the file was corrupted. In this case, I would get all good emails out of that folder and into another for safe keeping. Then delete all the messages you do not want. Then compact the folder to remove all traces of 'marked as deleted' email leaving an empty file ready to receive new mail. Why move all good emails first ? because if that file is corrupted then you could lose everything in the folder including good emails. '''If they were downloaded: '''Those emails must be on the server. If you use a product like CCLeaner to remove old temp files etc then it is possible you have not told it to exclude any Thunderbird file or folder. The program could have deleted your 'popstate.dat' file. The 'popstate.dat' file keeps a record of everything downloaded. If it were deleted and you keep copies of thousands of emails on server then they would get downloaded again. Info on deleted emails: Any email you delete will be 'marked as deleted' and hidden from view. This does not mean it is actually deleted from the file, after all you might delete in error and need to retrieve it. This is why you compact your folder after deleting several emails. Compacting is the processes of cleaning up the mbox text file removing all the sections with 'Marked as Deleted' emails. This is when they are truely deleted. Info on compacting - what it does and why it is important http://kb.mozillazine.org/Thunderbird_:_Tips_:_Compacting_Folders