Don't use dynamic DNS for the relay host FQDN. If you need DNS-based email load balancing, use a DNS round robin. This configuration means that the relay host name resolves to multiple IP addresses in a single lookup. The local firewall rules are created for all resolved IP addresses.
For example:
You want to configure DNS-based email load balancing to use addresses 172.16.165.40 and 172.16.165.33 for the relay host:
relay.test.local.
Under the
Email Connector Sending Email section, configure the following:
- Relay Host Value: relay.test.local
- Port: 25
- Domain: test.local
In this setup, don't configure dynamic DNS for
relay.test.local so that it resolves to either 172.16.165.40 or 172.16.165.33 in a single lookup. Configure your local DNS server to resolve the FQDN
relay.test.local to both 172.16.165.40 and 172.16.165.33 at one forward lookup as follows:
admin@ws40:~$ dig relay.test.local +short
172.16.165.40
172.16.165.33
admin@ws40:~$
With
relay.test.local resolving to multiple IP addresses, the ATD/IS back-end system creates permissive firewall rules for both 172.16.165.40 and 172.16.165.33 at the same time, allowing outgoing an SMTP connection to both IP addresses.