Customer email set to html, but sending plain text

Whenever I send an email to a customer, the email is sent in plain text even though I have specified in my configure->email options for the email to be sent in html. The administrator email however is sent in html. What I want is for all emails to be sent in html format. Here are some of the specifications that I have on my server.

PHP Version 4.4.1
MySQL 5.0.54
Zen Cart 1.3.8a
HTTP Server: Microsoft-IIS/6.0

Configure->Email Options:
Use MIME HTML when Sending Emails: true
Send Emails: true
Email Archiving Active: true
Email Admin Format: HTML
Email Transfer Method: SMTP

Configure->Customer Details
Customer Default Email Preference: HTML

It's not how the email is being SENT, but how the customer has chosen to RECEIVE it.

When shoppers sign up, they can select TEXT or HTML as the format. If they register and choose TEXT, then that is how they will get their emails - irrespective of you sending HTML.

Existing customers can toggle between TEXT and HTML in their "My Account" screen.

Furthermore, if their e-mail client software is set to receiving as TEXT, then there's nothing you can do about that.