Those who do not have the time to read lengthy messages may totally ignore your message if they see it's too long to read and, thus, may miss an important point you're trying to make. Address the issue at hand and refrain from personal attacks. If you disagree with a point someone has made, address that point rather than the individual. If you must use sarcasm, make it clear that that's what you're doing. Do not flame someone because they've made typographical or grammatical errors.
People do make mistakes.
Spelling or grammatical errors may very well be inadvertent and caused either by the individual rushing to get the message out or by their unfamiliarity with the editing capabilities of their software. Many people will ignore such messages rather than read them. Use an informative subject line. Your recipient will be more likely to read your message if the subject line catches their attention. If you decide to append a signature to your e-mail or bulletin board message, keep it brief.