The first thing to clarify is that there is a clear difference between writing an email in English to a friend and writing a formal email, whether it be for work, business, or in any case addressed to someone with whom you have little or no familiarity.
The purpose of an email to a friend is to stay in touch, to share the latest news about us, and to ask the other person to do the same: after all, you're talking to someone dear to you, so it's only right that you communicate with the same trust and affection you would if they were right there in front of you.
It's also true that, unlike a spoken conversation or a cell phone message, emails require a very specific structure, which must be followed even when sending an informal message to a friend.
So, let's continue reading to understand how to write one the right way, without distorting your way of communicating, maintaining an informal tone but, at the same time, respecting the pre-established "rules" required by email.