See hidden details in Gmail emails
The email you see in Gmail isn’t really what the original email looks like, at least not the email your email program interprets when you get it. Instead, there is a hidden source code that you can tap to see some additional information not included in the regular message.The source code of an e-mail shows the e-mail header information and often also shows the HTML code that controls how the message is displayed. This means you can see when a message was received, the server that sent it, and more.

Note: You can only view the full source of an email if you are using the desktop version of Gmail or Inbox. 
The mobile Gmail app doesn’t support viewing original messages.
How to View the Source Code of Gmail Messages
Open the message you want to view the source code for.
Find the top of the email where the subject, sender details and timestamp are located. 
On the right is the Reply button, then a small down arrow – click that to see a new menu.
Select Show Original from that menu to open a new tab showing the source code of the email.
To download the original message as a TXT file, you can use the “Download Original” button.  Alternatively, click Copy to Clipboard to copy all the text so you can paste it anywhere.

How to View the Source Code of Your Inbox Emails
If you use Gmail’s inbox, follow these steps:
Open the email.
Find the three-dot stacked menu button in the upper right corner of the message. Note that there are two of these buttons, but the one you’re looking for is at the top of the message itself, not the menu above the message. In other words, open the one located next to the email date.
Select Show original to open the source code in a new tab.
Just like in Gmail, you can download the full message to your computer as a text document, or copy the contents to the clipboard.