So the e0M is on the management VLAN per best practices. Are you implying that 100M is not sufficient for the ASUP traffic? Is there any way to manually set the route? It sounds like you are saying that the filer will take the shortest hop to the SMTP server…
What I mean is that you have to be careful that de default route does not use the e0M interface because it only operates at 100Mbps. So make sure that the default route is not in the range of the e0M interface.
If your SMTP server is in another IP range than the e0M interface, than you can check which route/interface is used by checking the routing table (command = route -s).
I usually disable the e0M interface and create an extra VLAN one of the 1Gbps interfaces just to make sure that in no circumstance the e0M interface is used for production network traffic.