DO... Use a descriptive subject line
There is nothing more annoying than receiving e-mails in your e-inbox with no heading, or a heading that does not explain what the content of the message is all about. When one receives multiple messages every day, the subject-line is important when reviewing and prioritising e-mail that is in one's mailbox. Also, if you include a descriptive title, your message is almost guaranteed to be read before the ones with blank or meaningless titles.
DO... Use opening and closing salutations
Some people have forgotten that e-mail is interpersonal communication between human beings. Basic civility still applies.
There is nothing much more impersonal than receiving an e-mail that doesn't at least say 'Hello...' or 'Hi...' for the opening; and 'Regards...' or 'Thanks...' or 'Take care...' or 'All the best...', or something similar as the closing.
DO... Use capital letters sparingly
The use of all-caps is shunned on the Internet. It's called SHOUTING. Every once in a while a word or two in capitals for particular emphasis is okay, but avoid overdoing it.
DO... Check spellings, grammar and format
Make a point to ensure that your e-mail is relatively readable. It doesn't have to be a work of art, but at least respect the basic rules of spelling and grammar. Most e-mail programs have a spell-checker option. Use it.
DON'T... Think that e-mail is instantaneous
Believe it or not, e-mail is not as reliable as a telephone call when it comes to timely communication!
The Internet is a loosely connected network of computers and telecommunications equipment owned, operated and managed by many independent companies, institutions and government organisations. Your e-mail must often travel a complex and circuitous route to get to its destination. For example, if someone schedules maintenance on a computer or a piece of equipment on the network that your e-mail must pass through, your message may be delayed and you won't even know it.