This CLI reads all mails from a variety of mailbox formats (maildir, mbox, babyl, mh and mmdf), then automatically detects, regroup and act on duplicates.

Duplicate detection is done by cherry-picking certain headers, in some cases doing some minor tweaking of the values to reduce them to a canonical form, and then computing a hash of those headers concatenated together.

Hashing headers#

In theory, we could rely on the mail’s Message-ID as a key to identify duplicate mails.

The reality is messier. There is no guarantee that Message-ID is unique or even present. Yes, certain broken mail servers which must remain nameless are guilty of this. 😩

That is why mdedup propose to identify uniqueness of mails based on an ordered hashed list of headers.

The list of headers to consider can be set with the -h/--hash-header option.

Mailing lists#

The default parameters of the CLI, especially the list of default headers have been crafted to limit the effects of mailing-lists on both the mail headers and body.

Mailing lists effects includes:

  • having an extra footer in the mail’s body, thus changing the Content-Length header;

  • mails with a new path described by the Received headers which would not be contained in any copy of the mail saved locally at the time it was sent to the list;

  • munging the Reply-To header even though it’s a bad idea;

  • adding plenty of other random headers which a copy saved locally at sending-time would not have, such as X-Mailman-Version, Precedence, X-BeenThere, List-*, Sender, Errors-To, and so on;

  • adding a prefix to the Subject header.

For added protection against accidentally removing mails due to false positives, duplicates are verified by comparing body sizes and also diff’ing the contents. If the sizes or contents differ by more than a threshold, they are not counted as duplicates.