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Mailman digest emails and threads
Mailman, Email

When replying to an email, the mail client adds a header field referring to the ID of the email you are replying to. Other mail programs will then use this to form threads.

Mailman, the mailing list software, doesn't include these references in plain-text digest mail, so when directly replying to a digest-email, the reply won't belong to the appropriate thread. To reply to threads propertly from a digest email, go to your membership options and change the digest to MIME format, where it will include proper mailto links with references. Ticking "Set Globally" will do it for other lists on the server you are member of.

Alternatively, if your mail host provides mail-filters, create a filter that moves all email from that list to a folder of your choosing, by matching the List-Id header. Then disable the digest mode.