Usually this occurs if something else is occuring and interupting Thunderbird. For example: In a new 'Write' message whilst typing. When it occurs, check your 'Drafts' folder to see if it has just saved the email in Drafts. If using Imap perhaps there was a pause whilst folders synchronised with server? Does it occur at intervals that seem to be when auto save to draft is occuring? It may also be your Anti-Virus program is temporarilly performing a check, scanning the uploading of files or downloading of email files ? Another program is consuming computer attention, such as OS updates or updates for other programs. Please read info on pros and cons on allowing AV to scan emails or Thunderbird profile folder. http://kb.mozillazine.org/Email_scanning_-_pros_and_cons