I could think of a few reasons for this. # An add-on is blocking any progress. Try start without add-ons enabled. Under Help you find an option to do that. Or have shift pressed down when you start Thunderbird. Do not mark any boxes in the popup, just continue. # Start Thunderbird offline by adding .offline in your thunderbirds start-up-line '''".... \Thunderbird" -offline''' EDIT That would eliminate any connection-problems. # Your folders are big and compact has been interrupted. That would leave temp-files called''' nstmp, nstmp-1 ..-2 ''' in your profile. those can be deleted. # You could also clear cache '''Tools / Clear recent history''' (shortcut ctrl+shift+del) # Your hd is near its limit and not enough free space for a copy of a folder can be found (See 3) A defrag might help. # EDIT It could also be your server that has a limit. # EDIT When you delete, a copy is made in trash-folder but the message isn't erased from the folder where it was. Its not until you compact it will be gone. So even if you delete without making a copy in trash (shift+del), you still need to compact and that doesn't work for you. You could make room by moving some old folders to another storage-unit