The ease of e-mail makes it ideal for casual correspondence, but it should be seen as a supplement to, and not a replacement for, other types of correspondence. Some situations demand a formal letter, a telephone call, or a handwritten note.
In this age of e-mail, a handwritten letter or note is still king when it comes to gracious personal communication. Sending a note of thanks or condolence by e-mail has the virtue of being an immediate expression of gratitude or sympathy, but it is not a substitute for sending a handwritten letter.
The rules of grammar and usage are not suspended for e-mail. Exotic punctuation, such as dashes, slashes, and dots, makes the copy hard to read and gives it a juvenile look, like signing a letter with Xs and Os. If your system has a spell checker, use it, no matter how good a speller you think you are.
Picturing your correspondence printed out and tacked to a bulletin board will help control the temptation to use e-mail gimmicks. Always use the subject line to let the reader know what the message is about. Tabs and centered or justified text can be lost in transmission. Type single-spaced with a blank line between paragraphs. Using all lowercase makes the message look trivial. All uppercase is equivalent to shouting.
If you want to forward a message, check first with the original author of the message. You also should get permission before passing around another person’s e-mail address; in addition, tell people where you got their address when you are sending them e-mail for the first time.
When sending e-mail internationally, keep the language formal. Casual language, slang, or jokes can confuse and/or annoy someone from another culture.