In the header, lines 1-4 are different for Gmail customers and Exchange customers. The images below demonstrate that difference.
First four lines of the header for a Gmail customer:
First four lines of the header for an Exchange customer:
Below are depictions of email headers so that you review the SenderBase Reputation scores (SRBS).
In the header, lines 1-4 are different for Gmail customers and Exchange customers. The images below demonstrate that difference.
First four lines of the header for a Gmail customer:
First four lines of the header for an Exchange customer: