Hi,
We have on our SmarterMail 16 set a hostname for our outgoing gateway.
In Settings -> Gateways / Failover -> Outgoing
I'm not sure here why you think you can only set an IP address.
However our gateway hostname points to a single host and is not round-robin to different hosts. I have no idea if this will correctly work. I am assuming it should if SM does the hostname lookup correctly.
Kind regards.
Sébastien Riccio
System & Network Admin
https://swisscenter.com