Thunderbird 38 introduced optional support for Maildir, which allows a single unique file per email (using the EML file extension), unlike the default monolithic single file format of one file per folder known as mbox. NOTE - this is NOT full Maildir in the sense that most users, particularly linux or mail administrators, know Maildir. You cannot point Thunderbird accounts to a mail server directory, nor do you get message flags stored with emails. See the wiki for more details.

Maildir is disabled by default because there are still many bugs. It is not 100% ready for users.

The remainder of this article explains the format change and how to test Maildir starting in Thunderbird 38.

Maildir is a major change for Thunderbird because it changes the mail storage mechanism. The single file per email storage format has significant benefits:

• Backups are easier and shorter because the amount of data to be backed up is reduced to only emails that have changed since the last backup.
• It is easier for anti-virus to scan emails in individual files, and infected mails can be removed without the loss of all the other mail in the folder, as often occurs today with anti-virus software which cannot handle multiple emails in one file.

How to turn on Maildir

To turn on Maildir:

1. Back up your email. If you don't know how to back up and restore your email and how to set an about:config variable, then you are not qualified to test Maildir.
2. Click Config Editor. You should get a warning about your warranty violation.

   
   

3. Click I'll be careful, I promise
4. Set the about:config variable documented in the Maildir wiki page.

Any email account created from now on will use Maildir. Existing accounts will continue to use mbox. If you want to use mbox again for new accounts, just reset the config variable.

Please file a bug if you encounter any Maildir issues. See also already filed bug reports.