When you reply a mail, you will find that most if not all mailclient put "Re:" before the subject. Some brainded mailclients even did this when it was already a "Re:" resulting in subjects like "Re: Re: Re: subject".

I sometimes get mail from even more braindead mailclients and server that are kind of localized and put "ANTW" ("antword", german for answer, read "reply") in the subject. Which will result in nice headers like "Re: Antw: Re: subject".

The most stupid thing is not that some mailclients are braindead and dont use internet standard. Thats only part of the problem, you can hardly call correct quoting a standard if the most used mail program (and hence users) dont get it anymore.

The most stupid thing is that the developers of the mailclients dont know what the "Re:" in the subject line means. It doesnt mean "Reply"!

See about.com:

"Re" is an abbreviated form of the Latin "in re", which, "re" being the ablative case of "res" (thing), means "concerning" or "in the matter of"

See also webster. So all you braindead localized mail developers, stop replying and start building something that is useful! Now comment on this with a funny subject line