Section 2 of RFC 2368 says that the body field is supposed to be in text/plain format, so you can't do HTML.
However even if you use plain text it's possible that some modern mail clients would render a URL as a clickable link anyway, though.
与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
