'Menu icon' > 'Options' > 'Layout' > select 'Folder Pane Columns' enable the 'Size' column - see image below as aid. What size does it say? After deleting several emails do you compact the folder to remove the marked as deleted emails? What size does it say is in the 'Junk'/'Spam' folder? You may empty it, but do you compact the folder? How big is the 'Sent' folder? Compacting - what it does and why it is important: http://kb.mozillazine.org/Thunderbird_:_Tips_:_Compacting_Folders If the emails are ones being returned because the recipient's server Inbox is full then you will get emails from server stating this. Some servers place a limit on the number/size of emails that can be sent within a given period. Gmail keep all emails in the 'All Mail' folder, so it acts as an archive. But it is possible to think you have deleted emails in eg: Inbox only to discover that gmail has still got a copy in the 'All Mail' folder which would cause you to get a message about a full quota.