> When the salutation in your letter or email starts with Hello or Hi, then you should put a comma before the name of the person you're addressing. It is also standard practice to put a comma after the name of the person you're addressing. ... You could also use an exclamation mark if you wanted to emphasize an emotion (like surprise). [1]
[1] https://www.grammar-monster.com/lessons/comma_with_dear_hell...
I'm not an expert but I've always seen it written this way and plenty of other sources suggest this practice. e.g. https://www.grammarly.com/blog/comma/