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.
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 was discouraged due to complexity or 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 it 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>
How to display an HTML email in a web application?
..most obvious answer is "put everything in a frame"...will it actually work?
Yes, e.g. Whiteout Networks GmbH's WHITEOUT.IO does it in /src/tpl/read.html and /src/js/controller/read-sandbox.js. Some of the security issues are handled by DOMPurify
..there are tons of issues..Is there a good, safe way..?
I know the message data format also under names EML or MHTML so looking for a good "XY to HTML converter" or "HTML5 document viewer with XY support" may point you to a usable results (e.g. GroupDocs.Viewer)
Some e-mail clients (e.g. GMail) don't use iframe, instead they use a mail parser (e.g. andris9/mailparser) and a HTML parser (e.g. cheeriojs/cheerio) to extract an e-mail-safe-html subset (see Stack Overflow: What guidelines for HTML email design are there? and Stack Overflow: Styling html email for GMail for some examples) or use a HTML sanitizer (e.g. Google's Caja, cure53/DOMPurify) and embed the code directly into the page.
But it is not always an easy thing, there is no consensus on what constitutes the e-mail-safe-html subset and you certainly don't wont to inline possibly infected attachments nor run anonymous CORS scripts within the secured user's session.
Anyway, as always, studying source code of various e-mail clients (see Wikipedia: Comparison of email clients) is the way to find out..
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.
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