No, the decision of the route for traffic to take is still based on its destination ip address, not on the traffic protocol - the same as it works for ISA Server. For example, the only way that you could target smtp traffic to use a specific interface is:
a) The specific interface is the external interface (the default) as designated within the Forefront TMG device;
b) You had used a smarthost within the mail server thereby giving a specific ip address that all smtp traffic was to be sent. This option then allows you to add a static route on the ftmg box stating that traffic for address ww.xx.yy.xzz use an alternative outbound interface.
