fallback_relay (default: empty)
Optional list of relay hosts for SMTP destinations that can't be found or that are unreachable. With Postfix 2.3 this parameter is renamed to smtp_fallback_relay.
By default, mail is returned to the sender when a destination is not found, and delivery is deferred when a destination is unreachable.
The fallback relays must be SMTP destinations. Specify a domain, host, host:port, [host]:port, [address] or [address]:port; the form [host] turns off MX lookups. If you specify multiple SMTP destinations, Postfix will try them in the specified order.
Note: before Postfix 2.2, do not use the fallback_relay feature when relaying mail for a backup or primary MX domain. Mail would loop between the Postfix MX host and the fallback_relay host when the final destination is unavailable.
* In main.cf specify "relay_transport = relay",
* In master.cf specify "-o fallback_relay =" (i.e., empty) at the end of the relay entry.
* In transport maps, specify "relay:nexthop..." as the right-hand side for backup or primary MX domain entries.
Postfix version 2.2 and later will not use the fallback_relay feature for destinations that it is MX host for.
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Here my actual scenario:
Servers:
mail1 - connected to mail2 and mail3 and internet with public IP
mail2 - connected to mail1 and mail3 and internet with public IP
mail3 - connected to mail2 and mail1 and internet with public IP
1. mail1 has its default smtp to its public IP since its a Postfix Server and its fallback_relay is mail2
2. The rest, same config, default smtp is it self, fallback_relay is the available mailx via intranet.
Config @ mail1
#/etc/postfix/main.cf
transport_maps = hash:/etc/postfix/transport
#/etc/postfix/transport
#smtp_server_mail2
@mail2.mydomain.com smtp_mail2:[mail2]
#
@mail3.mydomain.com smtp_mail3:[mail3]
#
#smtp_default, where all smtp goes thru
* smtp_default:
#/etc/postfix/master.cf
smtp_default unix - - n - - smtp -o fallback_relay=[xx.yy.zy.xz]
smtp_mail2 unix - - n - - smtp -o fallback_relay=[mail3]
smtp_mail3 unix - - n - - smtp -o fallback_relay=[mail2]
#
