Server level forwarding

I am having a problem with a specific outside contact in a group we have created, the email is being rejected.

The mail flow is like this;

• Bob sends to group1
• We get the email and process it, group1=frank and group2@outside.com
• Frank gets his email
• group2 gets a reject

The following organization rejected your message: mx1-us1.ppe-hosted.com. 

Diagnostic information for administrators:

Generating server: MBX1.supersecret.com

group2@outside.com
mx1-us1.ppe-hosted.com
Remote Server returned '554 5.7.1 group2@outside.com: Relay access denied'

So I contacted my spam filter vendor, proofpoint, and I got this reply;

Proofpoint Essentials does not support emails that are auto forwarded to external domains. In order to relay emails outbound through Proofpoint Essentials the sender needs to set up to be able to send outbound email. What happens in the auto forwarded email shows an external sending domain (gmail.com) is sending to an external receiving domain (OUTSIDE.com) and gets reject for relay access denied. 

Would you be able to set up a rule on your mail server for any auto-forwarded emails to go to the domain's mail server directly instead of using the smarthost back into Proofpoint Essentials? 

BlankMonkey