How do I view full email headers in different clients (Gmail, Outlook, etc.)?

Every email client hides full headers by default. Here's how to access them:

Gmail (web): Open the message. Click the three-dot menu in the top right. Select "Show original." A new tab shows complete headers and source.

Outlook (desktop): Open the message in its own window. Go to File > Properties. Headers appear in the "Internet headers" box at the bottom. Alternatively: right-click in the inbox, select Message Options.

Outlook (web): Open the message. Click the three-dot menu. Select "View message details" or "View message source."

Apple Mail: Open the message. Go to View > Message > Raw Source (or press Command+Option+U).

Yahoo Mail: Open the message. Click the three-dot menu. Select "View raw message."

Thunderbird: Open the message. Go to View > Headers > All. Or View > Message Source for complete raw format.

Once you have the headers, you can paste them into analysis tools like Google Admin Toolbox Messageheader or MXToolbox Header Analyzer for easier reading.