See https://github.com/Betterbird/thunderbird-patches/issues/125 for
reference.
This way, Rspamd will not add `FORGED_MUA_MOZILLA_MAIL_MSGID_UNKNOWN` to
mails sent perfectly find with Betterbird. Betterbird
(<https://www.betterbird.eu/>) is an adjusted version of Thunderbird,
fixing many bugs and adding long-wanted features. It is a common and
well-known alternative to Thunderbird, so I think the addition is
justified.