The headers of a message provide detailed information on the routing of a message and are an immense help in troubleshooting email-related issues. Because they contain more information than most would need or want, they are usually available but hidden.
Here is how to find email headers in some of the most popular email clients:
Procedure
Thunderbird
In the email -> Go to top-right corner, Click the "more" dropdown -> Select View Source
Apple Mail
Highlight the message -> Go to View at the very top -> Message -> All Headers
Outlook for Windows
Double-click the message so it opens in its own window >> Go to File->Properties >> Headers are displayed in the properties box near the bottom
Gmail
From the message, go to the upper right-hand side >> Click the 3 dots >> Select Show Original
Yahoo! Mail
From email message >> Top center next to "Spam", Click 3 dots >> Click View Raw Message
Outlook.com, Live.com, Hotmail
Click Message >> In message screen go to top right-hand side >> Click the dropdown >> Select View message source
Horde
Highlight email message >> Mid Screen Click + symbol >> View Source
Roundcube
Highlight email message >> Click the 3 dots with "More" underneath>> Click Show Source