I need to add a postfix transport entry, equivalent to sendmail's mailertable entries, such that for specific domains mail is routed through specific hosts instead of through what is found in an MX record look-up.
In other words, I need to implement what is described in this snippet from a standard /etc/postfix/transport file:
# When no transport is specified, Postfix uses the transport
# that matches the address domain class (see DESCRIPTION
# above). The following sends all mail for example.com and
# its subdomains to host gateway.example.com:
# example.com :[gateway.example.com]
# .example.com :[gateway.example.com]
# In the above example, the  suppress MX lookups. This
# prevents mail routing loops when your machine is primary
# MX host for example.com.
It looks like zmmta.cf might be the right place to do this such that it persists after restarts and upgrades, though I am not familiar with the syntax of that file at all.
Could someone explain how to do this?