An email may be short but it is still a letter. Do not forget this. Try not to let all your standards of literacy drop just because you know can create an email in seconds. Remember all your basic skill of spelling, punctuation and grammar. You can use accepted abbreviations, but
Don’t try to be funny and make your own up. This is as much a business communication as any other and you should always try to create exactly the same impression as you would wish to if you were writing a conventional letter. With friends, of course, it isn’t quite as important to use correct style, but even your best friend might find it a little odd that you suddenly don’t know how to spell.
Keep it short. No one really wants to read a huge, long message on the screen. If it has to be lengthy, then of course make it so, but consider, if this is the case, whether a letter on paper might not be a better option. Since it is going to take the recipient a while to study the contents, they might not read your message carefully for a couple of days anyway, so the whole purpose of email is invalidated.
Keep it up-to-date. You can send a letter about the price of a product only to discover that the currency exchange rate has changed in the last hour, invalidating your first message. So send another one- that’s what email is designed for.
Just because you can send it instantly doesn’t mean that an email won’t exist for ever, just as any letter on paper will. So think about what you really want to say and if you’re not sure, wait. Don’t just push the ‘send’ button because it’s there.
Your message may be read on screen, passed around someone’s office or filed away indefinitely. There’s no necessity to try to be humorous, super-casual or flippant just because you re using modern technology. If your message is so casual that you don’t care, then maybe you’re better off using the telephone, anyway.