Learn English – How to formally say in an E-mail: “Don’t call again”

It has been used that way, most markedly by AOL back in the early '00s, but is really isn't used that way, at least in the US, or at least in the parts of the US where I have lived.

Some people may understand your meaning, others wouldn't. If you were to ask me to "mail" something to you, I would be confused, and take a few seconds to catch on. But if you said "email", I would immediately know what you mean.

So ultimately, it's a matter of clarity. Use "email". It isn't saving you any time to say "mail" instead, and you probably save time, since you don't need to wait for the other person to catch on.

In the UK, black person is the usual way to describe someone of African or Caribbean ethnic background and I wouldn't expect it to be taken as offensive. Referring to someone as a black (as a noun) would be offensive.

Referring to someone as the black guy could conceivably be interpreted as a little disrespectful if you might have been expected to call them by name, depending on the context. In your specific example you could have said I don't remember your colleague's name but he's black, if that helps? and I wouldn't expect anyone to be upset by that form of words.

Your friend is either misinformed or engaging in propaganda against perceived "political correctness". Stories about the word "black" being banned in some context or other pop up in the tabloid press with depressing regularity but invariably turn out to be untrue or misreported.