Commas with direct address
“A comma is used to set off names or words used in direct address.
In correspondence, a comma typically follows the greeting, though a colon may be used instead (especially in formal correspondence).
If the greeting itself consists of a direct address, two marks of punctuation are needed (i.e., the comma in the direct address and the colon or comma following the greeting). (The first mark is often left out in casual correspondence.)
— The Chicago Manual of Style, 17th ed. ¶ 6.53