The email protocol is specified as text, so any email program that uses fonts, HTML or any other non-pure-text is a program not written according to the protocol.
What does this mean to you? It means that unless you control what email client the recipient uses (like a company-wide email sent to people whose computers you control), you can't be certain that what you send is what they'll receive - or that they'll receive anything. (One of my email filters is HTML - anyone sending me an HTML email is someone whose emails I wouldn't read anyway, so it gets deleted by the filter before it even gets to the email program.) Send HTML to some people and they'll just see the HTML tags, not what the tags are supposed to do, because their email program doesn't support HTML. (The protocol is plain text - the 127 characters and non-printing characters [like new line, tab, etc.] in the character set.) So instead of seeing your text in bold italics, they'll see <b><i>bold italics</i></b>.
Bottom line: Unless you know that all your recipients can receive the non-textual parts of your emails, send plain text. If you want to send fancy stuff, put it on a web server somewhere and email a link to it.