SMTP - SNAT - IP source

 X-Forwarded-for is used for HTTP Virtual Servers that have SNAT.

If you have an L4 Virtual Server it would be better to not use SNAT if you want to see the client IP.

There is no easy way to get the client's IP. As far as I know, you might try to have the client IP sent in a TCP header but then your server needs to be able to get it from that header.

Proxy Protocol Initiator | DevCentral

Or you might try to insert it in the payload, but again your servers need a way to get it.

Then you could try to use  npath routing

https://techdocs.f5.com/en-us/bigip-15-0-0/big-ip-local-traffic-manager-implementations/configuring-layer-3-npath-routing.html

But probably the best thing to do is to not use SNAT in this case.

For http is easier because most applications understand the x-forward-for header.