Problem:
You're onboarding a client who happens to be using code-two.
The issue is that by default, code-two acts as a smart host to sign the emails.
If you want to be able to use proofpoint in conjunction with code-two for outbound mail traffic, there's a special way to setup code-two so that email sent to code-two for signature insertion need to come back TO office365 for delivery out through proofpoint afterwards.
SOLUTION:
Code-Two has an article on their website explaining it ...
https://www.codetwo.com/kb/smart-host/