Whether they meant to be rude or not, avoid being rude in return. Respond with a kind answer or ignore their malicious comment but never stoop to their level. Someone's spelling is atrocious. Live with it. Everyone's not as good a speller as you are.
People are usually in a hurry on-line.
Cut them some slack. Someone posts something on a maillist with which you strongly disagree. Ignore it if possible. If it's something you feel must be addressed, kindly craft a response that sets forth your opinion without attacking the author of the original post. Remember that if you want the right to express your opinions, you must allow others to express theirs.