Performance layer 4 VIP for SMTP not balancing

Your assumption regarding expected behavior when a route is present is correct (assuming you are referring to a data-plane route).

I can think of half a dozen different ways to move forward, but all of them would require posting a lot of information with a lot of back-and-forth for troubleshooting. For that reason I'd recommend contacting support. For that reason I'd recommend contacting support. I'd be going through the same things they will, and they won't have you posting half your config to a publicly accessible site. :)

--jesse