MX Records: mydomain.tld. 33 IN MX 10 relay-1.mydomain.tld. mydomain.tld. 33 IN MX 20 relay-2.mydomain.tld.
with relay-1 and relay-2 having appropriate A and PTR records. Then in /etc/postfix/main.cf, I have relayhost = mydomain.tld I have seen this work with relay-1 unavailable. The outgoing message sat in the queue for a brief time, then went out through relay-2. I found a Red Hat solution https://access.redhat.com/solutions/4025391 that suggests using relayhost = [relay-1.mydomain.tld] smtp_fallback_relay = [relay-2.mydomain.tld] Comments ? Suggestions ?