Both. The greeting is a separate entity and despite ending in a comma or semicolon is not part of the first sentence of the body of the email. This is inherited

Both. The greeting is a separate entity and despite ending in a comma or semicolon is not part of the first sentence of the body of the email. This is inherited from English standards for writing letters on actual paper. Unless it's being graded for a class in school, is part of some official communication, or is going to a copy editor, though, it probably doesn't actually bother anyone if this convention is flouted.