[Mailman-Users] All recipients refused: (501, '5.0.0 Invalid domain name')

[Simon, imagine media]

No mail is being sent to any list members since moving to new server (redhat
7.1). I am getting "All recipients refused: (501, '5.0.0 Invalid domain
name')" in logs/smtp. I know this is some thing to do with sendmail/dns
settings, but cannot set the Dj option in sendmail.cf as this is a virtual
hosting server with hundreds of domains/emails. What other options / configs
should I test. All help appreciated as I am losing sleep!!

Simon