20. Do not request delivery and read receipts.
This will almost always annoy your recipient before he or she has even read your message. Besides, it usually does not work anyway since the recipient could have blocked that function, or his/her software might not support it, so what is the use of using it? If you want to know whether an email was received it is better to ask the recipient to let you know if it was received.

21. Do not ask to recall a message.
Biggest chances are that your message has already been delivered and read. A recall request would look very silly in that case wouldn't it? It is better just to send an email to say that you have made a mistake. This will look much more honest than trying to recall a message.
22. Do not copy a message or attachment without permission.
Do not copy a message or attachment belonging to another user without permission of the originator. If you do not ask permission first, you might be infringing on copyright laws.
23. Do not use email to discuss confidential information.
Sending an email is like sending a postcard. If you don't want your email to be displayed on a bulletin board, don't send it. Moreover, never make any libelous, sexist or racially discriminating comments in emails, even if they are meant to be a joke.
24. Use a meaningful subject.
Try to use a subject that is meaningful to the recipient as well as yourself. For instance, when you send an email to a company requesting information about a product, it is better to mention the actual name of the product, e.g. 'Product A information' than to just say 'product information' or the company's name in the subject.
25. Use active instead of passive.
Try to use the active voice of a verb wherever possible. For instance, 'We will process your order today', sounds better than 'Your order will be processed today'. The first sounds more personal, whereas the latter, especially when used frequently, sounds unnecessarily formal.
26. Avoid using URGENT and IMPORTANT.
Even more so than the high-priority option, you must at all times try to avoid these types of words in an email or subject line. Only use this if it is a really, really urgent or important message.
27. Avoid long sentences.
Try to keep your sentences to a maximum of 15-20 words. Email is meant to be a quick medium and requires a different kind of writing than letters. Also take care not to send emails that are too long. If a person receives an email that looks like a dissertation, chances are that they will not even attempt to read it!
28. Don't send or forward emails containing libelous, defamatory, offensive, racist or obscene remarks.
By sending or even just forwarding one libelous, or offensive remark in an email, you and your company can face court cases resulting in multi-million dollar penalties.
29. Don't forward virus hoaxes and chain letters.
If you receive an email message warning you of a new unstoppable virus that will immediately delete everything from your computer, this is most probably a hoax. By forwarding hoaxes you use valuable bandwidth and sometimes virus hoaxes contain viruses themselves, by attaching a so-called file that will stop the dangerous virus. The same goes for chain letters that promise incredible riches or ask your help for a charitable cause. Even if the content seems to be bona fide, the senders are usually not. Since it is impossible to find out whether a chain letter is real or not, the best place for it is the recycle bin.
30. Keep your language gender neutral.
In this day and age, avoid using sexist language such as: 'The user should add a signature by configuring his email program'. Apart from using he/she, you can also use the neutral gender: ''The user should add a signature by configuring the email program'.
31. Don't reply to spam.
By replying to spam or by unsubscribing, you are confirming that your email address is 'live'. Confirming this will only generate even more spam. Therefore, just hit the delete button or use email software to remove spam automatically.
32. Use cc: field sparingly.
Try not to use the cc: field unless the recipient in the cc: field knows why they are receiving a copy of the message. Using the cc: field can be confusing since the recipients might not know who is supposed to act on the message. Also, when responding to a cc: message, should you include the other recipient in the cc: field as well? This will depend on the situation. In general, do not include the person in the cc: field unless you have a particular reason for wanting this person to see your response. Again, make sure that this person will know why they are receiving a copy.

How do you enforce email etiquette?

The first step is to create a written email policy. This email policy should include all the do's and don'ts concerning the use of the company's email system and should be distributed amongst all employees. Secondly, employees must be trained to fully understand the importance of email etiquette. Finally, implementation of the rules can be monitored by using email management software and email response tools.