SMTP load balancing with Exchange 2010

I think the simple answer is what Nick brought up. It sounds like you have SNAT Automap enabled on the Virtual Server. This is often used as a quick fix for any routing issues. It makes it so your back end servers see the source IP of traffic flowing through the BIGIP as one of the F5's IP addresses. It would appear that in this scenario you want the servers to see the true client IP address. This will require disabling SNAT on the virtual server AND ensuring there are appropriate routes on your SMTP servers to route traffic back through the F5. Before tinkering with this one setting I would recommend reading over the Exchange 2010 deployment guide and ensuring you are following recommended practices with your Exchange deployment:

http://www.f5.com/pdf/deployment-guides/microsoft-exchange-2010-2013-iapp-dg.pdf