The header can help you troubleshoot email problems.

Select a message in the reading pane to open it in a new tab or window.

SelectView>Headers>Allfrom the menu bar to display the full message headers.

View headers in Thunderbird

To return to the standard set of headers, selectView>Headers>Normalfrom the menu.

The source code is more extensive than just headers and includes the coding for the entire email.

View > Message Source in Thunderbird