I once had a setup with 2 internet lines connected to a dual-wan router, which was connected to the external NIC of the SME. It was for pretty similar reasons as in your case: The DSL-provider, who was delivering email via smtp for me only had 16MB adsl. The faster provider had no open smtp server for delivering my emails.
This setup (it has to be configured in the router) was extremely messy and unstable. Can not recommend that. Either you put your whole email services out to the internet provider or you stick with sme for everything and your old one and only provider. That's what i did, to have all the advantages of SME.