HTML vs Plain Text
When sending emails you can include both a HTML and plain text version of the content.
By including both versions you leave it up to the customer and their email client to decide which format is to be displayed. This helps ensure two things:
Complete compatibility - certain mobile devices and other, older, email clients prefer plain text.
Customer friendliness - leave it up to your customers to turn off HTML formatting if they desire. At least they will still receive the quality emails you are sending!