WordPress sends e-mails in plain/text by default, and you can change that using the
wp_mail_content_type filter. However, if you change the content type to text/html for all your e-mails, you might break some messages that contain links, because WordPress puts <angle brackets> around links.
From the RFC2396:
Using <> angle brackets around each URI is especially recommended as a delimiting style for URI that contain whitespace.
If you’re going to change the content type for all your e-mails issued by WordPress, don’t forget to rewrite the default messages with proper markup. You can do that with various other filters, like
Finally, if you’re going to send e-mails in HTML, please make sure they all respond well to narrow screens on mobile devices, and preferably contain a plain/text version of the same message.