I’ve been using email on the Internet (and its predecessor, the ARPAnet) for 32 year. Here’s some advice from my experience.
The Prime Directive: Never send email when you’re angry. Never, ever. It always backfires and you always regret it. Trust me on this.
The rest of these recommendations apply primarily when you’re sending mail to anyone who isn’t a close friend.
Do not use sarcasm on mailing lists. Remember your tone of voice is not available to indicate that what you’re saying is sarcasm. Inevitably, a few people on the list will take what you say literally, and then you’ll have to underke the boring job of correcting everyone’s misimpression.
Be very polite. You almost can’t be too polite. Because your facial expressions and tone of voice are not present, it’s easy to write something that will seem demanding or commanding.
Know the difference between “Reply” and “Reply All”, and be careful to always use the appropriate one.
Be careful to address your mail to the right person! The automatic name-completion feature in many of the good mail clients can sometimes complete to a name that’s not what you expected.
Some people have separate home and company email addresses. Send personal mail to the home address.
Be careful about giving out someone else’s personal email address. Some people do not like to have their email addresses be well-known. So treat anyone else’s email address as if it were confidential information, until you get permission to distribute it.
When sending mail to many individuals, address the mail to yourself, and BCC it to everyone else. This way, the recipients cannot see the email addresses of the other recipients, thus protecting their privacy.
Make your subject lines descriptive and clear. If you’re replying, keep the same subject line (don’t worry about the “Re:”) so that mail readers can see which mail is grouped with which. If you’re communicating with friends, clever subject lines can be quite an art form and source of innocent merriment.
Save away all of your interesting email. It’s very handy to be able to refer to it when you subsequently communicate with the same person, or company.
Keep your email on your own computer. Leaving it on a net server is too much of a risk to your privacy. Even if you like Google (which I do) and trust them (which I pretty much do), you never know if conditions will change in the future, and by then it’s too late.
For very sensitive email, encryption is a good idea. Sadly, there isn’t an easy-to-use standard. I use the free version of AxCrypt from Axantum ; it’s only for Windows, unfortunately. There are plenty of others. Of course, the person to whom you are sending mail must also install the software, and you must have a shared passphrase. As long as you’re going to the trouble to encrypt, use a long passphrase for better security.
Please feel free to use the Comments below to add other good advice.