Resolve Spoofing errors for everyone at your domain name.
Traditionally, email discussion groups leave the sender's email address in the From header of the email.
A message to the Mailing List comes from the person that sent it, even though the Mail-List.com servers are delivering the email.
This confuses some ISPs, and they block the incoming email as being spam. They do this because they are not expecting email from their customer to come from a different server.
550-5.7.1 Your message has been rejected. The host which delivered your email is not listed as an authorized IP for the sending domain.
Change the Mail-List.com option to use the list address in the From header.
Add DNS records to your domain, allowing Mail-List.com to send emails from your domain name.