Email is a globally accepted tool for communication in professional circles just like any other medium which involves interaction between a large group of individuals, certain unspoken rules of etiquette are practiced which someone who is new to regularly using the medium for communication or even someone who might have been doing it for a while might not aware of.
The basic points of etiquette to be aware of while composing an email are the following:
A lot of people have the tendency to mark their emails as ‘Urgent' when the situation doesn't really call for it. This is frowned upon in professional circles and dilutes the promptness of the response you will receive when a message which is actually relevant is sent.
A sentence where all the words are capitalized comes off as you shouting your message. So, next time, if you feel the urge to do capitalize the words to convey the urgency of your message, don't.
Skipping the pleasantries or framing a message along the lines of "Hey, where's my money?" will not only let the person on the other side of the conversation know that you are immature but will also reduce the chances of further deals or new avenues of business with the same individual.
Replying to all
Replying to all is something which is not necessary often, so only do so if you think everyone on the list needs to hear the reply.
Avoid cc'ing a client on a mail where the boss might have said something about the client or a co-worker on an email chain where personal information might have been shared.
The first thought someone might have when they have been unnecessarily bcc'd on a mail is "who else has been bcc'd on other emails I received from this person?"
Sending an urgent email which requires a quick response in the middle of the night implies a certain mismanagement of time on your part. Not everyone is awake at odd hours in the night. This behavior might be overlooked once or twice but repeated occurrences will reflect poorly on the sender.
Unclear Subject lines
Obscure subject lines like "As discussed on Wednesday" or "Hey, it's me" will likely result in the email never being opened in the first place. On the other hand, an email with a clear and concise subject matter will most likely result in a prompt reply.
A professional email filled with LOLs and BTWs will most likely not only be frowned upon but will also reflect poorly on the organization that you represent.
At best, it shows you are unaware, a worst, it implies that you just don't care enough, but there is no excuse for sending a mail without running a spellcheck or reading through it.