When we send to a particular distribution list, we do not get an NDR and none of the recipients get the message. Below is the message flow that ends in "550 Administrative prohibition". The odd thing here is there is no sender "MAIL FROM:<>" is blank. I've contacted the admin of the other domain to see if he sees anything on his end or whether he can let these through.
220 mail.fakedomain.com ESMTP ready.EHLO webmail.mydomain.org
250-mail.fakedomain.com Hello ISP-IPaddress.com [x.x.x.x]
250-SIZE 52428800
250-8BITMIME
250-PIPELINING
250-STARTTLS
250 HELP
MAIL FROM:<> SIZE=2954
RCPT TO:<user1@fakedomain.com>
RCPT TO:<user2@fakedomain.com>
(there are about 15 recipients)
250 OK
250 Accepted
250 Accepted
250 Accepted
250 Accepted
250 Accepted
250 Accepted
250 Accepted
250 Accepted
250 Accepted
250 Accepted
250 Accepted
250 Accepted
250 Accepted
250 Accepted
DATA
550 Administrative prohibition
QUIT
