Very often, you hear people barking at someone because the poor fellow happened to send an HTML-mail to a mailing list. Now I personally do not understand this at all. Just about every email client out there, supports viewing HTML mails, and so do all the web based mail sites. And HTML mails are specially useful when you want to put certain text in bold, some text in italics, a couple of words in red or you need a bulleted list of items. They do add a few extra KB to every mail, but even a low-bandwidth guy like me has no problems with handling an extra 5-6 KB per email I receive.
It is true that HTML mails are abused quite often these days, where people embed silly smiley images, put JavaScript and do all sorts of other really dumb things. But using that as an argument against HTML mails is like saying that no one should be allowed to drive on the roads just because some drivers happen to be dangerous. HTML mails are a lot more handy than text mails, and the formatting capability is very useful when used with responsibility and maturity. Of course, when posting to newsgroups, it’s polite to use text mails, because there are still many newsreaders that do not handle HTML mails very well. But for regular emailing, I see no reason to avoid HTML mails. In fact I absolutely use HTML mails for all my emails, and even when I reply to a text mail. If someone thinks I am a silly idiot because I use HTML mails, then they are welcome to continue thinking so. It doesn’t concern me in the least.
Nish Nishant's technology blog 
I am not against Html, but why do you in Html you don’t have in RTF already?
The links? Forget, Outlook uses their own RTF-variant (I don’t remember the name of the file format, it’s MS proprietary anyway) to add hyperlink support to RTF. And there you go!
Hey Stephane
Yes, by HTML mails – I meant standard HTML mails, MSHTML mails (OE format), RTF mails etc.
HTML mail is not evil, but mail programs need to turn off all active elements like scriptng, applets, plug-ins, and ActiveX. In addtion, the user should be able to turn off images to avoid the exploitation of image “bugs” (image URIs with the mail address of the recipient) by spammers. In Mac OS X Mail, images will not be loaded for mail that is considered junk.
I think the best thing about HTML email is simple text wrapping and simple font changes; it’s true you can also this with RTF and text/plain format=wrapped, but HTML might actually be supported in more mail readers.
Good points, Robert. I specially think that all standard mail clients should give us an option to turn off displaying images in html mails.
I think it is courteous to reply to text emails in text format, maybe the person sending that text mail has a reason for doing so. Also text mails are easier to print, better for viewing on PDAs and when cut-and-pasting code from an email into VS.NET you don’t get all the HTML fragment crap.
It isn’t a big deal though, use what you like while being courteous.
Good point regarding PDAs, Paul. I never thought of that one.
Nish