My mail server was rejecting mail sent from your server due to the helo being issued by your server at sMTP time of just 'joindin'. It should be a fully qualified hostname.
Feb 24 11:13:33 oak postfix/policyd-weight: decided action=PREPEND X-policyd-weight: NOT_IN_SBL_XBL_SPAMHAUS=-1.5 NOT_IN_SPAMCOP=-1.5 NOT_IN_SORBS=-1.5 (only DNSBL check requested); <client=22.214.171.124> <helo=joindin> <email@example.com> <firstname.lastname@example.org>; delay: 1s
Feb 24 11:13:33 oak postfix/smtpd: NOQUEUE: reject: RCPT from 67-207-134-75.slicehost.net[126.96.36.199]: 504 5.5.2 <joindin>: Helo command rejected: need fully-qualified hostname; from=<email@example.com> to=<firstname.lastname@example.org> proto=ESMTP helo=<joindin>
I can obviously make my own server less strict, but I suspect you'll encounter other people with the same restriction.
It ought to be a configuration option within postfix/exim/whatever you're using - and hte obvious fix would be to change it to "joind.in"