The purpose of having email etiquette is to make communication effective.

Be polite
Address the recipient's name (Hello John, Hi John, Dear John) to prevent being seen as arrogant. This is also to ensure the recipient knows that the email is really for him/her. Some emails are sent by mistake.

Address the given name and family name correctly. For 'John Doe', it is inappropriate to write 'Dear Mr. John' or 'Dear Doe'. It should be 'Dear John' or "Dear Mr. Doe'. Take note that name of East Asian such as Chinese and Japanese has family name on the left.

Do not start a sentence with 'Kindly'. 'Kindly' is not the same with 'please'. 'Kindly', although by surface looks good, is however more to commanding the recipient to do something or sarcastically asking the recipient for help. Use 'Could you' instead of 'Can you'.

Do not use all capital letters unless you really mean to be arrogant.

Avoid using exclamation mark, especially on the subject line.

Do not use all small caps to avoid being perceived as not being serious in the interaction with the recipient. Work email should be formal.

It is alright to write email when being angry but the email should be saved as draft first and only decide to send it or not in the next morning. On the other hand, when receiving this kind of unfriendly email, do not reply in a harsh way and quickly but wait.

Avoid negative language. 'Please attend the meeting so that we can solve the problem' is better than 'If you do not attend the meeting, we cannot solve the problem'.

Avoid setting request for email delivery receipt. This gives unpleasant feeling to most recipients. The function may not work anyway due to recipient blocking it or recipient's email tool not supporting the function. After all, emails nowadays very rarely will become missing in delivery.

Try use neutral language gender. For example, use 'chairperson' instead of 'chairman'.

If you receive an email (especially external one) asking you for action but you feel that it is really not within your scope, do not just ignore the email. Instead reply and
a) thank the sender for the feedback.
b) say that you have conveyed the message to the related person (of course you must have done that)
c) ask the sender to liaise with the person you suggest. Give the person's scope in the matter mentioned in the sender's email.
d) in PS (post scriptum) ask the sender in future not to contact you again on the same nature of matter and write your scope. The font size of this PS may be decreased to make it meeker.

For effective communication
Write a specific subject that concerns the recipient even if it is a forwarded email. Be specific. If help is needed today then 'Please help today' can even be added in the subject line so that others will not think it is just another email for his/her info only and he/she opens the email a few days later. Nevertheless, it should not be too long until it is displayed as two lines in inbox view.

Avoid short forms. Not everyone understands them.

Include enough information. For example when getting a recipient in Malaysia to send in something by 0900 CET, then 'by 1500 Malaysian time' should be written. Doing some due diligence means helping yourselves.

Mention one topic per email. If there is another thing to talk about, write a new email.

Try write in point forms. Forget about what has been taught in school in writing in paragraph.

When asking a question, tell the recipient the things that you already know so that he/she will tell you only the things that you want to know. Being specific in the question will help too. An answer that is not an answer wastes time.

Not only all questions in the email should be answered, but also to take one step ahead to provide answer for question that you predict / preempt to possibly come after. By doing this, the recipient does not have to waste time to write another email again. Furthermore the recipient will be impressed with your efficiency.

Read the email you write before sending to minimize chance of mistake. At the same time you should also try break any sentence you feel that is long into short ones.

If you can see the person face-to-face to communicate what you want to say, then it is better not to send email. If someone sitting near you does send you this email, do not reply but walk to his / her place to discuss about the matter.

If you want to request for something with email, send it to one recipient only regardless it is internal or external email. For example you want response from the supplier regarding your complaint. If you put a few recipients in the email, most likely you will not get response at all because those few persons may think one of them will deal with you. If you really want to send it to a few persons, then send out a few copies with one recipient per email.

Do not just forward emails and expect other people to read the email and to reply you. For example if a client sends an email, the sales person should add constructive comment (say summarizing it a long email) before forwarding the email to other department to take action. You can forward email without any comment only if you are the boss of the person that you are about to forward to.

Send email only if it is truly necessary. Recipient's sense of importance towards your email decreases if he or she gets many.

Redundancy
Work email is generally used to request for or give info. For example reply with just "Thanks", "Noted" etc should not be sent.

Avoid putting disclaimer clause if it is not compulsory in the company. You are still responsible of the content of the email anyway even if there is a disclaimer clause.

Avoid putting full name card (company name, company registration number, phone number, fax number) for internal email. It is unpractical to put company website address because the recipient should know it from your email address (unless they are different from each other).

Avoid slogan or motto. It is flamboyant.

Use 'please' instead of 'I will be grateful if you' or 'you should' in front of your request. This is about politeness too as discussed above.

Avoid norminalized verbs (nouns that come from verbs, for example 'suggestion' is formed from 'suggest') because they usually need more words to say. For example 'Please suggest how to solve it' is shorter than 'Please give suggestions on how to solve it'. Norminalized verbs are also weaker than their root verbs.

Capitalizing words as you like is redundancy too. People tend to pick up the habit of using capital letters after getting into corporate world. "The auditors meet the auditees" is better than "The Auditors meet the Auditees".

Others
When there are five emails total in a chain thread, break the chain. Pick up the phone / stand up to resolve the issue.

Avoid attaching big file to the email. For example use JPG, GIF or PNG format instead of BMP to send a picture. Nobody likes to receive big file. If there is a shared drive, put the file in it and create a link in email to the file instead. Also you can be sure that the recipient will have the latest file if you still want to update the file right after sending the link.

Do not use 'cc' (courtesy copy or carbon copy) if it is not important to that person. And do not assume it is important to that person. You must have experienced getting work mails that make you wonder why you get those anyway. Information overloads today so you should do the courtesy of filtering info for others instead of sending 'courtesy copy'.

Do not use mass emails for information that is not needed by everyone. Select applicable recipients.

If the sender is a contact point of the company to an external party (e.g. a customer) and there is some information given by the sender's colleagues to be passed on to the external party, the sender should review it first to avoid sensitive information leakage. Do not just forward it because the person forwarding it will still be liable. By doing this the sender will gain overview of things that may help in the sender's work. Practice due diligence.

If you have two things to ask a recipient to reply, you may want to consider writing one thing first and wait for the reply. After getting the reply, send another. Most of us will tend to reply a smaller email more quickly. But if you have ten things to ask, do not send ten emails.

If you think that two persons may be able to help in your problem but one is more knowledgeable than the another, avoid sending one email to both because they may wait for each other to reply to you. Email the first one first, if you cannot get the person's help, then ask another.

Avoid expressions that show company weakness. Such expressions are not constructive. For example if a customer expects a written reply for a project complaint, do not write something like "After the investigation, we find that our internal processes are flawed". I do not think the customer wants to know the company's problem. Even worse you may be seen as giving excuses. Instead write action plans, for example "We will deploy a person immediately to solve your problem". In another example, if your colleague has done a mistake in the email communication and the mistake affects the customer, while you reply to the customer, do not correct the colleague in the same email. Correct him in a separate email.

Avoid expression that may be misinterpreted into infringing competition / monopoly law, for example "We dominate this market" or "Our customers have no choice."

Avoid using work email to forward emails unrelated to work, even news, out of the company. The consequence can be severe, especially when the material is sensitive, for instance a political article. Even if you forward such emails only internally, avoid sending to colleagues without their consent. What you think is important frequently turns out to be just nuisance for others.

Write 'Please see attachment' instead of 'Please find attachment'. Is the attachment hidden that the recipient needs to 'find' it?

When emailing the people outside the company, always strive to have correct grammar, zero typo, consistent, correct capitalization etc to preserve company's professional image.

Unless your job is mostly expecting emails, turn off the new mail notification (popup etc) to avoid focus stealing.