Help needed with SMTP iRule

An iRule will not assist you in this context. As long as SNAT is involved your exchange administrator will only see connections from an IP address on the F5 device.

 

You have a few options here.

 

You can leave the F5 in place and setup remote logging via syslog to have the F5 record details of each connection to a database.

 

You can leave the F5 in place, setup a routing virtual server, and configure exchange to use the F5 as their gateway. You could then disable SNAT and exchange will see real connection IP Addresses.

 

You can stop using the F5 to load balance SMTP traffic to exchange, and leverage DNS round robin instead. (not a great option)