Is JavaScript Supported in an Email Message

How to send an email from JavaScript

You can't send an email directly with javascript.

You can, however, open the user's mail client:

window.open('mailto:test@example.com');

There are also some parameters to pre-fill the subject and the body:

window.open('mailto:test@example.com?subject=subject&body=body');

Another solution would be to do an ajax call to your server, so that the server sends the email. Be careful not to allow anyone to send any email through your server.

How can I add javascript to an the email?

Don’t count on it. Most if not all mail clients will refuse to run Javascript within an e-mail.

If you want to include buttons, insert images with different links to a public web page of yours. Also, do include the button images within the mail message itself (and reference them by Content-ID), since many mail clients nowadays even refuse to load remote images.

This is exactly what Google Calendar does. Of course, Gmail recognizes invitation attachments (text/calendar), and so provides its own interface for the yes/no/maybe buttons.

javascript not working inside email

All email clients have (and must have) a strict policy against running any Javascript found in emails.

This is a strong security requirement, you will not get around it. Which is good. You can only place Javascript on the target site, not in the email.

Use Javascript to create an HTML email in Microsoft Outlook

MSG file format is documented, but it is certainly not fun...
Why not create an EML (MIME) file?

The suggestion is to use the EML (MIME) format. According to the OP, they considered the MSG file format (#4), but were discouraged by its complexity and lack of JS libraries that process that format. If MSG file was considered, MIME is a much better choice - it is text based, so no special libraries are required to create it. Outlook will be able to open it just as easily as an MSG file.

To make sure EML message is treated as an unsent message by Outlook, set the X-Unsent MIME header to 1.

The simplest EML file would look like the following:

To: Joe The User <joe@domain.demo>
Subject: Test EML message
X-Unsent: 1
Content-Type: text/html

<html>
<body>
Test message with <b>bold</b> text.
</body>
</html>

Can I add a Javascript function in C# to a gmail e-mail?

For security reasons, email clients will strip all script and most CSS from emails before displaying them.

You can't do that.

Sending emails with Javascript

The way I'm doing it now is basically like this:

The HTML:

<textarea id="myText">
    Lorem ipsum...
</textarea>
<button onclick="sendMail(); return false">Send</button>

The Javascript:

function sendMail() {
    var link = "mailto:me@example.com"
             + "?cc=myCCaddress@example.com"
             + "&subject=" + encodeURIComponent("This is my subject")
             + "&body=" + encodeURIComponent(document.getElementById('myText').value)
    ;
    
    window.location.href = link;
}

This, surprisingly, works rather well. The only problem is that if the body is particularly long (somewhere over 2000 characters), then it just opens a new email but there's no information in it. I suspect that it'd be to do with the maximum length of the URL being exceeded.