is comma applied after addressing To in a letter? Please give the source also.?

Answer 1

For formal letters, use a colon:
To so-and-so:

For informal letters, use a comma:
Dear so-and-so,

Answer 2

no. to is the one to whom it is being sent. it has an adress and name. the use of a comma only occurs for the firsyt time when you salute the recipient such when you writew dear sir, dear madam, dear sam. and then the comma is right after as shown in this sentence .
dear sir,
dear madam,
dear sam,
to whom it may concern,
these are the kinds of salutations yuou may find. each is followed by the comma then the next sentence is started using a capital letter as the first letter of the firsyt word.
this is standard letter format. depending on whether it is a business or personal letter has no differences in format but only in how you use the paragraph indents. sometimes you use an indent, sometimes not. if you don't know what format you want, get a book on letter and other writing styles.

Answer 3

It can either be:

To: Mr. Flabberghasted
or
Dear Mr. _____, (<---comma)
but you don't put
To, Mr.______