A friend of mine uses both an exclamation mark and a comma in his letter salutations, like so:
I have never seen this before, and was wondering if it's grammatically kosher. Thoughts?
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The first thing to say is that punctuation isn't grammar. The second thing to say is that punctuation is there to help the reader and if it doesn't do that it serves no purpose. In your example, either the exclamation mark or the comma would be helpful. The two together most certainly are not.