@Duck59 certainly, I agree with what you write. The less formal and more colloquial language is proportionate to the degree of knowledge and even more to that of confidence. Not with all my friends (even people with long knowledge and confidence) I have the same way to address and vice versa, because then intervene the character, the nuances, the moment, the context, time. Then there is confidence and confidence. I like people who do not need to say "you are taking too much confidence" but that they know how to understand for themselves what can be said and when and what not or not at that time.
But when you are dealing with someone we know little or do not know at all, regardless of the context and the way in which you communicate, then I think it is obvious to maintain a certain distance but with a friendly attitude.
For work I often write mails or rather pragmatic messages because it lacks material time and the motivation to be more narrative. When, instead, years ago I worked in the Personnel Management of a company, communication, especially that via e-mail (including commas) was fundamental and sometimes dangerous. Commas became the essence of communication ... a stress that never ended.
But if someone I do not know writes me with a smile (which, however, by e-mail is understood), I answer him with two