Thunderbird has had hidden support (Maildir is disabled by default) for the Maildir mail storage format for several years, but Maildir is still not 100% ready for users. This article explains the format change and how to test Maildir starting in Thunderbird 38 (Maildir is not ready for testing prior to version 38).

Maildir is a single file per email (EML), unlike the current default monolithic single file format, which is one file per folder (known a mbox). This storage format has significant benefits:

• Backups are easier because the amount of data needed to be backed up is reduced to just those 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.

This is a major change for Thunderbird because it changes the mail storage mechanism. The development team have spent a significant amount of time developing maildir.

However, due to a lack of time and testers before Thunderbird 38 was released, the actual underlying changes have not had the extensive manual testing that would normally occur for such a major change.

How to turn on Maildir

Here's how to turn on Maildir:

1. Backup your email. If you don't know how to backup your email, then you are not qualified to test Maildir.
2. Click the Application menu button .
3. Click OptionsPreferences and then OptionsPreferences.
4. Click Advanced and then click General.
5. Next to "Message Store Type for new accounts", select File per message (maildir).
6. Any email account created from now on will use Maildir (existing accounts will continue to use mbox). Please file a bug if you encounter any Maildir issues.