Be sure to address the first name and last 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'. This happens quite frequently in Asian countries.

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. We should also use 'Could you' instead of 'Can you'.

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

Avoid using exclamation mark, especially on the subject line.

Do not use all small caps. Work email should be formal. We should show that we regard the interaction with the recipient seriously.

It is OK to write email when we are angry but we should save it as draft first and only decide to send it or not in the next morning. And when we receive this kind of email, do not reply in a harsh way.

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.

Avoid using the word 'urgent' or 'important' in the subject line. What is important to you may not be important to the recipient.

Use neutral language gender.

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. You may decrease the font size of this PS to make it meeker.

For effective communication
Write a specific subject that concerns the recipient even if it is a forwarded mail. Be specific. If we need help today then we can even add 'Please help today' in front of our subject line so that others will not think it is just another mail 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.

Avoid short forms. Not everyone understands them like you do.

Include enough information. For example we want a recipient in Malaysia to send us something by 0900 CET, then we should write 'by 1500 Malaysian time'. Doing some due diligence means helping ourselves.

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

Try write in point forms. Forget what the teachers taught us in school that we should not write in points.

If we ask a question, tell the recipient the things that we already know so that he/she will tell us only the things that we want to know. Being specific in the question will help too.

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

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

If we can see the person face-to-face to communicate our words, then it is better not to send email. If someone sitting near us does send us 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.

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 our company. We 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. Putting email address under our name is even more unpractical. Likewise with company website address as the recipient should know it from our 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 our 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 we like is redundancy too. We 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, we should break the chain. Pick up the phone / stand up to resolve the issue.

Avoid attaching big file to the email. For example we should 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. I think everyone has experienced getting work mails that make us wonder why we get those anyway. Information overloads today so we should do the courtesy of filtering info for others instead of sending 'courtesy copy'.

If we are a contact point of the company to an external party (e.g. a customer) and there is some information given by our colleagues to be passed on to the external party, review it first to avoid sensitive information leakage. Do not just forward it because we will still be liable. Doing this will also help us gain overview of things that may help in our work.

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 we may be seen as giving excuses. Instead write action plans, for example "We will deploy a person immediately to solve your problem".

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 we forward such emails only internally, avoid sending to colleagues without their consent. What we 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?

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