It seems when sending a message to another user References: and In-Reply-To: headers are incorrectly included or at least with non-unique identifiers in the form of

correspondence-123-456@musicbrainz.org

Whereas 123 is the recipient's and 456 is the sender's user ID.

Which results in email clients such as Thunderbird treating all such messages as part of the same thread.

My Thunderbird threading settings:

mail.strict_threading true
mail.thread_without_re false
mail.correct_threading true

References:

• https://datatracker.ietf.org/doc/html/rfc2822#section-3.6.4
• https://wiki.mozilla.org/MailNews:Message_Threading
• https://github.com/metabrainz/musicbrainz-server/blob/v-2024-05-13-schema-change/lib/MusicBrainz/Server/Email.pm#L98-L105