I've posted one that is similar but you can't send files from it and it doesn't support HTML.
This site allows you to send an email to someone, looking like its sent it from somewhere else. Example:
Go to the site, plug your information
From: [email protected]
To: [email protected]
Subject: You need to send me your TabooCrime
Attach: Download me =P
Text: Download the attachment
SOOM.cz - Fake Mailer
I posted this for informational purposes only, and not to actually be used. HeyaEveryone nor my real identity is responsible for any illegal actions you may take using the site I linked, or the example I used.