The optional sender dependent addresses file allows the relay host for for outgoing SMTP connections to be set on a per-address or per-domain basis. The left hand side of each entry can be either a full email address like `foo@bar.com`, or just a domain like `@bar.com`.

The right hand side must be in the format transport:nexthop. In the typical case where you want to relay mail via a specific mail server, transport should be `smtp` and nexthop the hostname or IP address of the mail server.