Your email is your business card.
Title of the email - emails without a title or with a title that says nothing can be ignored or taken frivolously. It is always a good idea to put in the title of the email and a specific one - announcing the content of the message.
Beware of "forward"! Forwarding to other recipients the content of received emails is a serious business faux pas. It can bury our relationship with a client for good, especially if the content of the emails includes personal comments or company secrets. Concise content - leave elaborations to academics.
Business emails should be specific, concise and clear. It is worth opting for visibly separated paragraphs or bullets of the email content - it will be not only legible, but also aesthetically pleasing.
Form adapted to the recipient - it is important that the phrases of politeness and greetings correspond to the relationship you have with your client. To write more or less formally? This is something we already have to decide for ourselves.