hebig.org/blog
Personal webnode of Haiko Hebig

If Mozilla Mails gets new messages from a server and disk space runs out, you are in trouble. Mozilla stops the download, is unable to properly close the mailbox and summary files and throws an error that reads like: "An error has occured in your boxname.msf file. You might need to close down Mozilla and delete the specified file from its location." Do not do so before backing up the boxname and boxname.msf files. If you delete boxname.msf and restart Mozilla, it will overwrite your boxname file, that is, kill your mails. Do not expect that Mozilla rebuilds the summary file from the mail file.

In my case, I could open mails younger than those that has been in the preview pane when the error occured, but older mails have been lost and additional mails could not be received, even with free disk available. Deleting boxname.msf fixes the latter problem.

So make your sure you always have plenty of free space on your Mozilla profile drive or share, do good profile backups and manually backup the boxname and boxname.msf files before deleting the .msf file from the profile folder. Important messages received after the error can then be extracted manually from the boxname file in your texteditor.