PROBLEM:
You used the proofpoint Integrated deployment but External email invite forwarded internally are being Quarantined by Proofpoint
SOLUTION:
Go to Exchange Admin Center
Edit Rule Condition for the PPE Inbound Redirect
Add the following exception:
Add Except if
Except if
"Message header ."
"Matches these text patterns"
"Enter text " >> "X-MS-Exchange-MeetingForward-Message"
"Words" >> "Forward"