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.
-- Detect forged Mozilla Mail/Thunderbird/Seamonkey/Postbox headers
-- Mozilla based X-Mailer
local user_agent_mozilla5 = 'User-Agent=/^\\s*Mozilla\\/5\\.0/H'
-local user_agent_thunderbird = 'User-Agent=/^\\s*(Thunderbird|Mozilla Thunderbird|Mozilla\\/.*Gecko\\/.*(Thunderbird|Icedove)\\/)/H'
+local user_agent_thunderbird = 'User-Agent=/^\\s*(Thunderbird|Mozilla Thunderbird|Mozilla\\/.*Gecko\\/.*(Thunderbird|Betterbird|Icedove)\\/)/H'
local user_agent_seamonkey = 'User-Agent=/^\\s*Mozilla\\/5\\.0\\s.+\\sSeaMonkey\\/\\d+\\.\\d+/H'
local user_agent_postbox = [[User-Agent=/^\s*Mozilla\/5\.0\s\([^)]+\)\sGecko\/\d+\sPostboxApp\/\d+(?:\.\d+){2,3}$/H]]
local user_agent_mozilla = string.format('(%s) & !(%s) & !(%s) & !(%s)', user_agent_mozilla5, user_agent_thunderbird, user_agent_seamonkey, user_agent_postbox)