We use the sender_dependent_relayhost_maps configuration to relay outbound mail for some of our users that have a 3rd party secure mail service. The configuration is as below:

/opt/zimbra/postfix/main.cf:
sender_dependent_relayhost_maps = hash:/opt/zimbra/postfix/conf/bysender

/opt/zimbra/postfix/conf/bysender:
@whateverdomain.com [XXX.XXX.X.XXX]

postmap bysender
postfix restart

Everything is working fine but we were wondering if we can have multiple ip's in the configuration, separated by comma or semicolon, to list fallback servers.

Google didn't help me much with this one.