The References header is an ordered list of mails, and some clients
only look at the last mail in the list to determine to which mail the
mail is in reply to.
Therefore, to be a less obtrusive as possible, put the fallback
address first.
Some info about the reply key lookup in the References header:
https://docs.gitlab.com/ee/administration/reply_by_email.html#how-it-works
Microsoft Exchange would append a comma and another
message id into the References header, therefore we'll
need to fallback and parse the header by ourselves.
Closes#26567