Inbox is really designed to be an inbox for new/recent messages and not a general purpose storage unit :) If you have emails downloaded to Thunderbird (not just headers) then the emails are stored in one single text file. They are written one after the other to the file in the order downloaded. You could imagine this like one single document and each email is like a paragraph. The Inbox can also get a lot of activity in the form of deleting unwanted emails. When you delete an email, it is hidden and marked as deleted, but it may still be in the file. This keeps a file larger in size than you may expect. Compacting the file removes those 'marked as deleted' emails. So to use same analogy, the 'paragraph' emails get marked and then removed when compacting. So if there was any corruption in that file or if an Anti-virus did not like it for some reason and messed about trying to fix it or quarantine it, you could stand to lose a lot of emails; especially as imap accounts are also synchronised with the server. So it is more risky. Super large text files may also be difficult to open using a text editor. Not that this occurs very often, but it is downright pain if your really need to do it. Compacting means initally copying the file in same location before removing the unwanted sections, but this may become impossible if too large and there is insufficient space to do it. You can set up imap account to 'expunge' Inbox each time you exit Thunderbird. This will auto compact the Inbox. Located in Account Settings > Server Settings for the imap mail account. Helpful info: * http://kb.mozillazine.org/Thunderbird_:_Tips_:_Compacting_Folders * http://kb.mozillazine.org/Nstmp_folders * http://kb.mozillazine.org/Performance_%28Thunderbird%29 * http://kb.mozillazine.org/Keep_it_working_%28Thunderbird%29

Thanks, I'll set about slimming it down then.