If this is a POP account, then Thunderbird has determined that your 'Sent' folder has reached it's maximum size. If it is an IMAP account, the message may be from the service provider. Each mail folder contains active messages and messages that have been marked for deletion. If you have not 'compacted' the folder, the deleted messages it contains still count against the total size of the folder. Only when you compact that folder, are the deleted messages actually removed from it. The folder is re-written and the file size is reduced. If the folder has been compacted lately and is still 'full', your still have the option to either 'move' older messages into a new folder that you create under 'Local Folders' or to 'archive' the folder which does a similar function. It moves older messages into a series of folders that have a consistent structure based on your preferences. https://support.mozilla.org/en-US/kb/archived-messages <blockquote> Related questions:<br /> ''"The folder Sent is full, and can’t hold any more messages."'' https://support.mozilla.org/en-US/questions/1058922 <br /><br /> ''"says sent box full all the time - but it is empty - cannot refer to any sent emails"''<br /> https://support.mozilla.org/en-US/questions/991855 </blockquote> TB-38.3 Win10-PC