postfix: backup relay in transport map

[pactoo]

Hello,

A postfix mail relay in the DMZ sends outbound mails to the default smarthost. For the inbound domains, there is a transport map, where I define the next hop for each domain.

However, for inbound mails, how do I specify an alternative/back relay in the transport map, for the case when the first one is not alive? DNS is not available, so the MX trick does not work and I need to sprecify a hostname as relay and another hostname for that backuprelay.

I have two domains: internal.one and internal.two. When everything is ok, mails for internal.one get forwarded to smtp.one and mails for domain.two get forwarded to smtp.two. Mails for every other domain are sent to the outside smarthost: smtp.my_isp.net

Nothing dramatic so far. However, if, for what reason ever, smtp.one fails, mails shall get forwarded to smtp_backup.one and I have no idea how to give the transport map that information. Pretty much all information I find about this topic is MX centric. Maybe someone here has a similar setup that solves this issue without MX

Thanks