Is mentioning full name on email replies considered rude? [closed]

In casual and friendly conversations, people normally call each other by first names (or don't use names at all when the reference is clear). If you choose to use their full name, "Mr. Mishra", or a title like "Doctor", it suggests that you do not consider the conversation to be friendly. If this occurs in an online setting that's normally assumed to be casual, it will often be interpreted as confrontational. It's a common technique of trolls in online forums, replying to other posters with full names to annoy them.

Parents will sometimes use a child's full name when calling for them in order to discipline them. This is somewhat related to the above situation -- using a full name and a loud voice distinguishes it from the normal case where they would use first names or pet names. A wife might also do this when confronting her spouse (maybe that's sexist, but I've never seen it the other way around), calling them by a full name instead of "dear" or "honey".

In more formal situations, there's no special connotation to using full names. It's normal in news stories and introducing lecturers at conferences. In business correspondence, usually you would use an honorific and last name, e.g. Dr./Mr. So-and-so.