BodyText property represents a text-only version of the mail message and doesn't support any kind markup to differentiate links from other parts of the text. However, some (but not all) e-mail clients are known to recognize links that start with "http://" and display them as clickable links.
BodyHtml property represents a HTML version of the mail message and supports all kinds of HTML markup, including links (like
<a href="url">text</a>). Nearly all modern e-mail clients support this.
BodyText can be present in a single mail message at the same time. In that case, HTML-capable e-mail clients (a majority) will display the HTML version and legacy/text-only e-mail clients will display the text version. This means that when sending e-mail that contains links, using both BodyHtml and BodyText is recommended.