SMTP Relay Whitelist

I have seen similar questions in the past. You already have the 2 possible solutions, default gateway to F5, or filtering with iRules. The filtering could be done with AFM if you have that.

I don't know all the SMTP headers, but my understanding is that any server reference there is with name and not IP. If you know a header that you can change to add the original IP, that can work.

You just need to do a TCP collect and change the payload.

https://devcentral.f5.com/wiki/irules.tcp__collect.ashx

However, I would assume that the access list on the server side is done when the connection is established. The payload content (SMTP data), is only looked after the connection is established, so no effect in this case.