The absence of commas in greeting expressions (or other direct addresses) has become so common, so that I started to think that it is correct for English language (unlike in Russian). I have googled the topic and quickly realized it is not true: there must be a comma in direct addresses, except the cases when we use an adjective before a name of a title, for example 'Dear John'.
Incorrect: Hello man
Good morning Alice
Correct: Hello, man
Good morning, Alice
The mistake is being made, even by English natives, so often, that I have decided to start this topic. Any thoughts or comments?
English is a descriptive language. If something becomes so widespread, it becomes acceptable. So while you're technically right, dropping the comma has become so widespread that it has become okay. People won't even notice if you drop it. I actually notice when someone uses the comma, not when someone doesn't.
Here's what the Chicago Manual of Style has to say on the topic:
So here’s the official Chicago ruling: Use a comma before direct address and choose appropriate punctuation after it. In e-mail greetings, do as you please.
Here's what Grammar Girl has to say:
The problem is that almost nobody knows that greetings should be punctuated this way, so it looks weird when you do it right. In fact, it's extremely rare to see an e-mail salutation that uses a comma after the hi. I'm always torn about whether to use the comma. It is correct, but it seems a bit pedantic given the widespread use of the incorrect alternative – especially when you are replying to someone who has already done it the wrong way. Use your own judgment. I usually put it in, but you'll be in good company if you leave it out.
Dear Squiggly, (correct)
Hi, Squiggly. (correct)
Hi Squiggly, (widespread to the point of becoming acceptable)