Help! How do I send an autoresponse HTML email?

Need to send an autoresponse HTML email when a visitor fills out a form

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I have a customer that has a form on their website where they want to be able to send a HTML email autoresponse after a visitor fills in the form and hits "submit". I am currently using formmailPHP to handle the form submittal.

I have the HTML email complete and ready to deliver, I just can't figure out how to do this.

The host I am using has an option to do an autoresponse from an email account, so I am already doing this for each form submittal, however the problem is they only support text email responses, not HTML.

Any ideas? Your help would be greatly appreciated!