If posting to Newsgroups, be aware that many are archived, and the archives are available for a very long time. Don't say anything that might come back and haunt you years down the track. It is generally not possible to retrieve messages once they have been sent. Messages should be concise and relevant to the group. Don't post messages to inappropriate newsgroups.
Spelling mistakes tend to be the rule rather than the exception.
Questioning someone else's spelling is not good netiquette, and not necessary unless it occurs in a critical place, like a possibly mis-spelt URL Don't get involved in or respond to Flame Wars. If you find a newsgroup or topic offensive, avoid it. Sending harassing messages to the group is not only frowned on, it may be illegal. Keep private messages private, don't post to the group as well. Above all, respect the culture of the group.