SMTP SNAT vs iRule header

Hello everyone,

I have an issue related SMTP.

This is the incoming mail topology in the organization:

 Internet ---> FW ---> LTM ---> Cisco ESA ----> Exchange server

Between the LTM and the Cisco ESA (Email Security Appliance) is the configuration:

F5 - LTM

VS type: standard

Port: 25

Profiles: TCP, SMTP

SNAT: auto-map

Pool: contains 2 members representing the ESA servers 

LB method: Round Robin

==========================================================================

Cisco ESA

Configured to recognize incoming relay form the F5 self IPs according to the "Received: from" header.

==========================================================================

After looking at the email headers it seems that the "Received: from" header because of the auto-map.

Disabling the auto-map is not an option, according to my customer, because the F5 cannot be the ESA's default gateway.

Our other option is to try adding a new x-header containing the original IP of the sender.

Does anyone know if that is even possible to add an email header using an iRule?

Unfortunately didn't anything related in the F5 articles.