Every email client has settings that determine whether or not the contact views emails in HTML format or plain text format. Some of your contacts will have their email clients set to only display emails in plain text. For this reason every email sent from our system must contain a text version.
The HTML email must closely match the text email. If they do not closely match, the email is likely to be flagged as spam.
As a default, your HTML version will be displayed for those contacts who have HTML enabled.