Set SMTP IP translation.

In the virtual server configuration in the GUI, there will be an item under the configuration section on the properties tab called 'Source Address Translation' with a dropdown box beside it. Just set this to none and then click 'Update' at the bottom.

Note that the SMTP server will need to have a route back to the client(s) through the BIG-IP in order for this setup to work without any kind of SNAT.