Every email client has an option to view the headers of a message or, in other words, the source code of the email. Headers are essential to diagnose delivery problems or to find out why an email was flagged as SPAM.
Below we explain how to view them depending on the email client you use.
Roundcube
Open the message, expand the "More" menu and click "Show source".
Horde
Open the message, expand the "Other options" menu and click "View source".
Gmail
Expand the email options (three-dot icon) and click "Show original".
Hotmail / Outlook
Expand the "Actions" menu and click "View message source".
Yahoo
Expand the "More" menu and click "View raw message".
If you are looking at the headers to understand why an email was flagged as SPAM, also check the guide Emails received as SPAM.