The optional transport file specifies a mapping from domain hierarchies to message delivery transports and/or relay hosts. The mapping is used by the trivial-rewrite(8) daemon.

The format of the transport table is as follows:

- `domain` mapped to.. `transport:nexthop`: Mail for domain is delivered through transport to nexthop.
- `.domain` mapped to.. `transport:nexthop`: Mail for any subdomain of domain is delivered through transport to nexthop.

Note: transport map entries take precedence over domains specified in the `mydestination` parameter. If you use the optional transport map, it may be safer to specify explicit entries for all domains specified in mydestination, for example:

- `hostname.my.domain` ... `local:`
- `localhost.my.domain` ... `local:`

The interpretation of the nexthop field is transport dependent. In the case of SMTP, specify `host:service` for a non-default server port, and use `[host]` or `[host:port]` in order to disable MX (mail exchanger) DNS lookups. The `[]` form can also be used with IP addresses instead of hostnames.