Hi Simple things first. 1) When you 'delete' a smaller message, does it go to the Trash folder? Have you emptied the Trash folder? 2) Have you compacted your folders as you delete the old messages? (Right click folder - choose compact.) As you are cleaning out (deleting) old messages, you need to compact to actually free up space. Just deleting does not create usable space. 3) Have you tried deleting with [shift] delete? This deletes immediately without moving the message to trash. You still need to compact from time to time. 4) If your space really is that full, try this - but I suggest you start on a small folder. First delete a bunch of the smaller messages, then compact. After compacting, you should be able to delete slightly larger messages. Again, delete then compact. This will free up enough space to start getting to the larger messages, and then work on the Inbox using the same method. 5) If Thunderbird's trash folder has become corrupt you may need to follow the procedure here https://support.mozilla.org/en-US/kb/cannot-delete-messages Good luck, Agnes