Is it correct to use a capital letter after the comma in Dear sir?

No, it is not correct to use a capital letter after the comma in "Dear sir."

Here's why:

* "Dear sir" is a salutation. It's a polite way to address a person in a letter or email.

* Salutations are not sentences. They don't have a subject or a verb, and they don't require a capital letter.

Correct: Dear sir,

Incorrect: Dear sir,