In general, not bothering people by e-mail. Not sending pointless chain-letters, not giving away friends' e-mail addresses, etc.
When guaging the opinion of your recipient, keep in mind that you should do for others as they have done for you. The content they send out is the content they most appreciate. Strongly contrasting content may be questionable -- valueless or even offensive. If sent only polite notes by them, return only polite notes. If sent lewd jokes by them, then it's probably fine to send them such content. Keep in mind that this must be separated by email address. That is, you must treat each email address like a new person. Your friend may be a different person while at work than while at home. If in doubt as to what content is appreciated or frowned upon, ask before sending.
Enforced Conduct Code
The recipient may have certain legal or enforced social requirements. If they are at an office, there is certain content which is not office-safe. This varies widely and could be any combination of topics surrounding sex, drugs, violence, or even swearing. Even if the person is not at an office, they may not appreciate being sent the latest cracker's handbook. While some content should be reasonably obvious, in this matter it is generally best to ask before sending material.
The recipient must be judged before some email should be sent. A rappore must be created between you and that person so you can judge if they would appreciate certain stuff. While perhaps embarassing.. if in doubt, ask!
This varies widely because each recipient has unique tastes. Let us start with varying kinds of "humour". Some people don't want any jokes, or certain potentially offensive jokes, disgusting or extreme things, etc. Some content can be considered valueless because it has no use for the recipient. Sending a recipe to a cook might be considered valueless or even insulting to some.
- Re-Mailing - Passing a chain letter to a friend
- Signatures - Most users would know of signatures as those advertisement tags which are added by 'free' email services. Because of their pervasiveness they are generally accepted. However, be aware that the content of a signature is still subject to the suggestions applicable to the body of the email. Profanity or other 'innapropriateness' may be frowned upon, depending on the recipient's preferences or requirements.
- Subject - content
- Purportal.com - When you are forwarded an email, check to see if it's a known chain letter before passing it on.