SMTP Faxing - Multiple Domains

It appears as though it is not possible to define more than one Domain for a Fax node when using SMTP based faxing. Is this the case ?

The reason multiple domains is required is that in the one R/3 system, different countries may have different fax service providers so separate domains are required to route the smtp-fax to the correct destination.

This is possible under an RFC based fax connection. The std country routing schema can be pointed to the appropriate RFC fax device. Why not under SMTP?

Any help appreciated