Mozilla Thunderbird is a cross-platform, feature-rich free email client with many customization and security options. If you want to view the HTML source code of an email message, Mozilla makes it easy, whether it's an unopened or open message.
How to set up Google Messages in Mozilla Thunderbird to view and send messages (SMS) from Thunderbird
If you want to determine the origin of a spam message or troubleshoot an email, it can be helpful to view the source of the message.
Mark an unread message in your Thunderbird inbox.
From the top menu, select View > Message Source.
