All of these are fine to use, but only in the right place. They are not general-purpose phrases. Use them if you haven't heard from someone in a long time or didn't expect to hear from this person for some other reason. If you have a regular e-mail correspondence with a person, ask a question, and get a reply ten minutes later, "good to hear from you" (or any of the others) would be out of place.how to acknowlegde a mail...
also works fine in friendly written correspondence.
It's rare that someone asks a question and literally quotes the direct answer to his own question.How about email?
Ah, but it is.It is not in written form.