[FIX] mail: less confusing To: header for mailing-list posts

Many mail clients will replace the name in the To:
header with Me if the To: email matches the email
of the user. These users will see To: Me instead of
"Followers of ..." and usually believe this was a
private email from the sender to them.
But when replying they would reply to the whole list.

Fix this by explicitly forcing the To: to be the
mailing list address.