SMTP authentication interface request

We need to configure SMTP authentication for our external customers on our SMG appliance.

The SMG interfaces are:

Inbound: 192.168.1.1 -> 25
Outbount: 192.168.2.1 -> 25

The documentation says:

"You must use an IP address/port combination for SMTP authentication that is different from both your inbound and outbound IP address/port combinations."

So, is it possible to define this interface on the authentication Tab: 192.168.1.1 -> 587 ?
It is not the same combination than the inbound interface, the port is different, right?

Or, do we need to add virtual IP to do this?

Thank you for your answer.

Best regards.