by Ken Task.
Did the move also include a domain name change? Or just an IP address change? It might be that however your EMail distribution is setup, messages are being rejected because of either of the above and the reply-to address is in need of changing.
Check the reply-to address for all outbound EMail from Moodle - Site Admin menu - Plugins - Message Outputs - Email.
"Emails are sometimes sent out on behalf of a user (eg forum posts). The email address you specify here will be used as the "From" address in those cases when the recipients should not be able to reply directly to the user (eg when a user chooses to keep their address private)."
'spirit of sharing', Ken