How many emails have you got in the inbox? The Inbox should be used as an Inbox for new incoming mail and not as a general storage. * Create new folders to organise mail and move emails into those folders. * delete old unwanted mail, As your Inbox may well be littered with 'marked as deleted' mail, on this occasion, I would organise emails and completely empty the Inbox before compacting. because if there is any corruption, you could lose a load of emails. When you have completely moved everything out of the Inbox. * empty the Trash folder * empty the Junk folder. * compact all your mail folders using 'File > Compact folders' Do you Archive anything older than eg: a couple of months ? If no then I recommend you use this. You can set up Archive Options and then archive mail. Suggest you archive by month and select to keep exisiting folder structure. * https://support.mozilla.org/en-US/kb/archived-messages The emails will still be visible in Thunderbird, but it will organise mail into smaller folders. '''Background info:''' In your Folder Pane you see folders that contain emails. In reality, these 'folders' are actually mbox files. Emails are stored in the order they were downloaded, one after the other in the Inbox file. They are not stored as separate .eml files. So, any emails in the 'Inbox' folder are actually stored in one single file. So if anything happened to that one single file, you risk losing a lot of emails. Hence why you can create Folders in the Folder Pane to organise emails and use Archiving, so storing them in smaller batches. '''Compacting - what it does and why it is important.''' When you delete an email, it is 'marked as deleted' and from your point of view it goes from eg: Inbox to Trash In reality, it is still in the Inbox, just 'marked as deleted' and hidden from view. So it is taking up space. Compacting will remove all 'marked as deleted' mail, so freeing up valuable space. * http://kb.mozillazine.org/Compacting_folders