Nik, in SmarterMail v16 the outgoing gateway selection was handled on a priority basis. So you would need to add the outgoing gateways and associate them with the same priority of mail you configured the domain for that you want to be passed through it.
In the latest release of SmarterMail, gateway selection can be configured on a per-domain basis directly, and does not require configuration of the priorities.
Kyle Kerst
Lead Internal Network/System Administrator
SmarterTools Inc.
smartertools.com