Email HTML Source - Is it possible to edit the HTML source of the email before sending?

NULL

Which part of the source code? If you are referring MIME as a whole, than no.

I need to edit both html and mime SOURCES in an email client

The OP ( Andrewed1 ) clearly asked for editing the HTML source, NOT the MIME source.

In OE (Outlook Express) it was easy, convenient, fast, powerful, to edit the HTML source (“OE > Message > New Message > View > Source Edit” will bring the 3 tabs “Edit - Source - Preview” at bottom of that New Message window).

And, still in OE, while a little less direct it was as well very powerful to edit the MIME source (Drag the message from OE Message pane to Windows Explorer will create an EML file that you can edit in Notepad and drag back into the Message pane); the whole (HTML + MIME) made OE very efficient for handling email in a very clean, pretty and effective way, with messages small in volume and easy to read while full of useful information and properly indexed hence properly threaded even if NOT so when received).

Versailles, Wed 08 Jan 2014 18:59:00 +0100

Hi, you can add HTML if you click in “New Message” window on Toolbar - Insert - HTML, or right click into text field - Insert - HTML.

Jan