What punctuation mark is polaced after the greeting?

Typically, a comma is placed after a greeting if it is part of another sentence.
Example:
Hello, my name is Brian.

Or, it can stand alone with a period if it is not part of the next sentence.
Example:
Hello. My name is Brian.

In a formal business letter, a colon comes after the greeting.
Example:
Dear Mr. Brown:

In an informal letter, a comma comes after the greeting.
Example:
Dear Mr. Brown,