It’s standard practice to put a salutation on its own line above the first paragraph of body text, with a gap such as a line space between them.
When letters were hand written and space was short, it was common to add a comma at the end of the salutation. In today's word processing environment, we no longer need a comma between the salutation and the text because the line break is sufficient.
For the same reason, you don’t need a comma after your sign-off a the end of a letter or email.
Dear Mr Greenway
I am writing about ...
Yours sincerely
Bina Goldberg

