[Tanialoves]

I am teaching my students to write letters now. Explaining some cases of punctuation, I say that they should write
- Hello, Mike,
- Hi, Kate,
- Good morning, Mister Smith,
- How are you, Toni,
I explain that we always use a comma when directly addressing someone/something, regardless of whether the direct address is at the beginning or end of the sentence. If the direct address is in the middle of a sentence, we use a pair of commas to set off the direct address.

But, I often receive emails (from native speakers) that begin "Hi Tania," or "Hello Tania,".

So, what is the correct punctuation? What should I teach my students?

I do appreciate your help.