What does the mailto beside the sender's email mean?

Question

John sent an email to Tom, who sent me an email, embedding the original email. In Gmail, this is the message Tom sent me:

Hi I'm tom.
-----Original Message-----
From: john@example.com [mailto:john@example2.com]
Sent: 1 January 1111 11:11
To: tom@example.com
Cc: mary@example.com;
Subject: Re: subject heading here

I wanted to send a message John. I do not have John's email, however from the email that Tom sent me, I can see that John's email is john@example.com.

The problem is that beside john@example.com, there is a [mailto:john@example2.com].

So to email John, should I email john@example.com or john@example2.com ?

To rephrase the question, what does the mailto beside the sender's email mean?

Answer 1

"mailto:" is a URI scheme that indicates that the following text is an email address. So it helps computers to understand the semantics of the text.

This is in the same way that "http:" is a URI scheme that indicates that the following text is an address of a site that can be accessed using the http protocol.

Most OSs will recognise a mailto: URI and know that if you click it, that your default email client should be opened with a compose window and the text following the mailto: populated in the To: field. Ie, you click the address and it your email client opens ready to write them an email.

While it is difficult to see exactly why the From: address and mailto: addresses are different, it is more likely that the mailto: is right, and probably stems from a misconfigured email client.