I have Postfix 2.1.5 set up on a Debian 3.1 server. On two occasions mail has looped back to the sender because the 1st MX record for the recipient resolved to an invalid host.
e.g. A message to someone@BlueH2Ogroup.com looped back to the sender, because tcsmg1.BlueH2Ogroup.com is non-existent.
rob@mini:~$ host -t MX BlueH2Ogroup.com
BlueH2Ogroup.com mail is handled by 10 tcsmg1.BlueH2Ogroup.com.
BlueH2Ogroup.com mail is handled by 50 mail.BlueH2Ogroup.com.
rob@mini:~$ host tcsmg1.BlueH2Ogroup.com
Host tcsmg1.BlueH2Ogroup.com not found: 3(NXDOMAIN)
rob@mini:~$ host mail.BlueH2Ogroup.com
mail.BlueH2Ogroup.com has address 126.96.36.199
How can I get Postfix to try the 2nd MX in the event that the 1st is unresolvable? [I believe it works OK, if the 1st can't be connected to. The problem is when the 1st does not resolve.]
Here are the MX-related bits of postconf:
root@mini:/etc/postfix# postconf | grep mx
ignore_mx_lookup_error = no
parent_domain_matches_subdomains = debug_peer_list,fast_flush_domains,mynetworks,permit_mx_backup_networks,qmqpd_authorized_clients,relay_domains,smtpd_access_maps
smtp_defer_if_no_mx_address_found = no
smtp_mx_address_limit = 0
smtp_mx_session_limit = 2