Changing your hostname which your mail server sends out

Occassionaly you have a internal mailserver on a machine which has a hostname like machine.example but for mail purposes your machine is known to the outside world as mail.example.com one can use the following helo_data on in your transports for exim to announce yourself as mail.example.com:

remote_smtp:
  driver = smtp
  helo_data = mail.example.com

exim router for relaying via a smarthost

User the "begin routers" section add:

route_append:
    driver = manualroute
    domains = *
    transport = remote_smtp
    route_data = "smarthost.host.name byname"