Multiple remote connections, return first responder?

We make one initial attempt to deliver mail internally in our application before tossing it off to an MTA to sit in retry-land. While you and I may both know that SMTP isn't real time, the vast majority of the unwashed masses have gotten used to the fact that it operates like a real-time service most of the time, and so we're trying our best to accommodate them, while still maintaining our sanity. Since it's very rare that this provider's primary and secondary MX's are unavailable at the same time, I thought the F5 might provide a nice solution whereby minimal app changes are required.