Message Rejected By Mail-List.Com Because Sender Does Not Have Reverse DNS Setup
Reverse DNS is a critical step in authenticating email senders, and all legitimate senders of email have this configured correctly.
if not, Mail-List.com will reject the email.
Reverse DNS means that the name of the host machine sending the email, matches up to the IP address that it is sending from.
See this article on WIkipedia for more information - http://en.wikipedia.org/wiki/Reverse_DNS_lookup
to verify that the mail server is set up correctly.
You will need to work with your Upstream ISP to resolve this problem
They have assigned you the IP address, and the need to update that IP address in - in-addr.arpa - to point to the your mail host name.
Common error messasges in this situation
host lookup failed (18.104.22.168 does not match any IP address for mail.example.com)
While the techie is fixing this problem, a workaround is to submit messages from our web site - https://database.mail-list.com/app/send-message
Changing yourlistname to the name of your list.