I haven't recieved any emails from NT's forum, so it looks like my theory is correct.
From the SMTP log file (I changed the sender and to: addresses to post here):
09:15:21 64.202.166.11 - - 250-pre-smtp28-02.prod.mesa1.secureserver.net+ 0
09:15:21 64.202.166.11 MAIL - FROM:<
[email protected]>+SIZE=1002 0
09:15:21 64.202.166.11 - - 354+go+ahead+punk,+make+my+day 0
09:15:21 64.202.166.11 - - 250+ok 0
09:15:21 64.202.166.11 RCPT - TO:<
[email protected]> 0
09:15:21 64.202.166.11 - - 250+ok 0
09:15:21 64.202.166.11 DATA - - 0
09:15:21 64.202.166.11 - - 354+go+ahead+punk,+make+my+day 0
09:15:21 64.202.166.11 - - 451+See+http://pobox.com/~djb/docs/smtplf.html. 0
09:15:21 64.202.166.11 QUIT - - 0
If you go to the pobox.com link it is clear that Threads is sending out bare linefeeds.
