Postix transport smtp failover

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http://serverfault.com – We have a multi-tenant email relay set up that has a transport map file that looks like this: domain1.com smtp:mail.domain1.com domain2.com smtp:mail.domain2.com domain3.com smtp:mail.domain3.com [etc] In the event mail.domain1.com is down, email for domain1.com will be held by the postfix server until mail.domain1.com starts responding again. However we have a customer who has a backup DSL line on their site, an their email server is also available over this. How can I tell the transport to failover to a different host if the first is unavailable? (HowTos)