Postfix transport map with failover entries?

Postfix relays a few domains to a fixed IP address, or its failover address.

1.2.3.4 and 11.12.13.14 (failover)

These two IP addresses are where the relay should occur.

Currently, transport_map

exemple1.com   relay:[1.2.3.4]:587
exemple2.com   relay:[1.2.3.4]:587

Is there a way, in case [1.2.3.4] times out, to use the failover automatically, like

exemple1.com   relay:[1.2.3.4]:587
exemple2.com   relay:[1.2.3.4]:587
exemple1.com   relay:[11.12.13.14]:587
exemple2.com   relay:[11.12.13.14]:587

Would that work?

(There is this question which is a tad different. Adding multiple IPs to /etc/hosts (or DNS if that was possible) would use either IP. In my case the failover should only be used if the main one times out)

Solution 1:

Excerpt from Postfix 3.5 release notes

[Feature 20200111] the Postfix SMTP and LMTP client support a list of nexthop destinations separated by comma or whitespace. These destinations will be tried in the specified order.

The list form can be specified in relayhost, transport_maps, default_transport, and sender_dependent_default_transport_maps.

Examples:

/etc/postfix/main.cf:
    relayhost = foo.example, bar.example
    default_transport = smtp:foo.example, bar.example.

NOTE: this is an SMTP and LMTP client feature. It does not work for other Postfix delivery agents.

If you don't have Postfix 3.5 you can specify the destination as a domain name without brakets - postfix will do MX lookups for that domain name and use MX to send the email.