There's a handful of important e-mail conventions that would make everyone's life a little easier if we could all try to follow them.
Use Meaningful Subject Lines
This easy practice will help keep you on good terms with even your most e-mail-swamped friends. A subject line can say so much, and many of us really like knowing if it's a joke, recipe, or some other non-urgent item that can be opened at our leisure.
Don't Type With ALL CAPS
Its been said too many times, but almost everyone finds reading text in all caps annoying, hence the reputation as online screaming.
Quote Select Parts Of A Previous E-Mail
One of the most common breaches of netiquette is the repeating of entire e-mails or postings when replying only to a small portion. When replying to a long e-mail it is best to highlight just the minimum needed of the previous comments to preface your response with. This avoids multiple-reply e-mail discussions where the messages get so long to be unwieldy and impossible to read.
Don't Spam
The worst online offense in most people's eyes is the dreaded unsolicited commercial e-mail. Don't let some marketing type talk you into the "power of online mass-advertising," you'll live to regret it.
Don't Pass Around Chain Letters
Another thing the Internet would be better off without is e-mail chain letters. They're not cute. Don't send them to your friends. Don't even send them to your enemies.
Don't Use Excessive Signature Files
It can be real useful to have contact information appended to your e-mail messages in a signature file. It can occasionally be cute to have a short, one-line quotation as your signature file. It is however unnecessary and bad form to use more than four lines for your signature file. Also signature files can get very tedious in situations where they are often repeated like newsgroup threads and multiple-reply e-mail discussions. In these cases, it's polite to omit your signature file after the first use.