Outlook 2013, 2010 (Windows): Double-click the message to open it in a new window. Select the File tab, and then click Properties.

Outlook 2007 (Windows): Double-click the message to open it, and then in the Options group, click the dialog box launcher (small square with an arrow).

Outlook Web App (OWA): Double-click the message to open it, and then click the Message Details icon (an envelope with a small document over it).

Outlook Express, Windows Mail, or Windows Live Mail (Windows): In the message index, right-click the message, and then select Properties. Click the Details tab.

Outlook 2011 (Mac OS X): In your Inbox (or other folder), right-click or control-click the message, and then select View Source.

Thunderbird (Windows, Mac OS X): Click View, select Headers, and then choose All.

Mail (Mac OS X): With the message selected, from the View menu, select Message, and then select either All Headers or Long Headers.

Gmail: Open the message in your Gmail inbox, Click the down-arrow in the top-right corner of the message, Click the “Show original” link toward the bottom of the options box. The message will open in a separate window with the full message headers at the top.

Yahoo Mail: Open the email message in your Yahoo Mail inbox. Click the “Full Headers” link located in the lower-right corner of the email message.