Unable to see the source IP address on SMTP VIP pool members.

The ideal configuration for back end servers is for their default route to take traffic back through the BIG-IP system so that the destination address/port translation that occurred on the inbound traffic can be "undone" on the outbound traffic. You can use the BIG-IP system's floating self IP address on the VLAN where your SMTP servers are as the default gateway for the servers. That will preserve the original client's IP address in the source field so that it is visible to the SMTP servers. SNAT (source address translation) is only required if, for whatever reason, you are unable to set the BIG-IP system as the server's default route.

X-Forwarded-For is an HTTP header only - not available in SMTP. See Kevin Stewart's response to a similar question here: https://devcentral.f5.com/questions/smtp-support-with-x-forwarded-for-in-one-armed-deployment