[saviorand]

Didn't know about all these headers. Is there a way you can check if a header is present in an email from a client? e.g from Gmail

[jivings]

Yes! You can look at the original source of the email.

Open the email in Gmail, click the three-dot menu in the top right, click "show original". A new tab will open with the raw email including all the headers.

[saviorand]

Wow, thanks. Live and learn. Does the way some tools check whether the email was read by the recipient or not have something to do with this structure, too?