What are Gmail email headers?
Each individual email consists of email headers. The email headers consist of tracking information that highlights the path the email took as it went through various servers. It contains information, such as the time stamps, IP addresses, sender and recipient information - which is useful to help troubleshoot any queries or issues you might have with your Gmail emails, such as emails not being sent, marked as spam, or bouncing incorrectly.

For more information, see how you can view Gmail email headers.
What is a HAR file?
HAR files contain sensitive data, including the page content you downloaded while recording and your cookies. This can allow anyone with the HAR file to impersonate your account and all the information you submitted while recording (including personal details, passwords, credit card numbers, and so on). Please be careful when you generate the HAR file.

For more information, see how you can generate a HAR file for troubleshooting.
What is Message Capture?

This is useful to identify the shape and content of a message before and after it has been processed by Exclaimer.

This feature copies messages that match the Message Capture conditions and email the diagnostic data to the specified diagnostic recipient. Message capture does not interrupt mail flow; captured messages will still be delivered to the intended recipients.

For more information, see how you can enable Message Capture.

What is an extended message trace?

This is used to trace emails through Exchange Online based on defined criteria (for example, an email address, date range, delivery status, or message ID). Often this is required for a support query or troubleshooting mail flow issues within Microsoft 365.  

Extended traces can sometimes take several hours to complete. You can check progress at any time from the Downloadable reports section on the Message Trace home page.

For more information, see how you can generate a Microsoft 365 extended message trace and How to generate a Google Workspace message trace.