In extension of the Three Cs of Communication by @Rose Eliff , which focus on the "sender" role..
The "Receiver" role in a communication process is at least as important as the "Sender" role. Hence...
Tip:
Whether in an email, voicemail or on conference call, when communicating with others remember:
Listen/Read
Comprehend
Take your time
Nowadays, people do not listen to understand, to learn, to just be informed etc.
They listen to answer, to react, to oppose, to argue etc.
They aren't what they is! (*)
They are what they do!
And that's bad!
(*) you are what you is (https://genius.com/Frank-zappa-you-are-what-you-is-lyrics)