What form of session persistence actually works with SMTP?

I have two Exchange Hub Transport Servers wanting to deliver SMTP mail to a LB pool of two Ironmail SMTP gateways. How can I insure persistent sessions to the gateways? I'm assuming Cookies don't apply to SMTP, which is the method I'm used to configuring. I tried source-addr, but since I'm SNATing the Exchange servers to the self-IP of the F5, everything goes to one gateway. What have you folks seen work with SMTP traffic to stick traffic to the same destination for each session?