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Hi john I miss you too.
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You actually need two commas, a semicolon (or a period) and a capital letter:

Hi, John; I miss you, too.
Hi, John. I miss you, too.

I suggest the second.
Comments  
AnonymousHi john I miss you too.
Yes, you should put a comma after John. The comma just adds a short pause, and a small separation between "Hi John" and "I miss you too". You could also use a period, which creates a longer pause and more separation between the sentences.
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 Mister Micawber's reply was promoted to an answer.
Mister MicawberHi, John; I miss you, too.
Hi, John. I miss you, too.
I respectfully disagree. I believe this is a STYLE issue, and I feel those commas are inappropriate.
I have seen this way of writing occasionally (I have a friend who uses it, and she is quite well-educated - she would never try to thoughtlessly split an infinitive), but I believe it was abandoned a long time ago (at least before I went to school, and I'm 42) simply because when you say that sentence, there are NO PAUSES to correspond to the commas. This is why I feel they are inappropriate. They may of course be grammatically defensible.

Of course conventions change constantly, and any of us can become frustrated when they see someone violating a convention that they hold dear. My pet hate is "anymore" but if I was 50 years older, I might be moaning about people who spell "today" without the hyphen!
I think we have an obligation to teach people the conventions used by modern literate people. But maybe I'm wrong about this.
KrisBlueNZ
Mister MicawberHi, John; I miss you, too.

Hi, John. I miss you, too.

I respectfully disagree.

I agree with you.

By the way, I don't think that commas always denote pauses.

But in any case, those sentences just don't need commas, in my opinion.

And they also look very cluttered with the commas.

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It is fine to express your opinion here, Kris, but not to the extent of belittling others'. I am one of those modern literate people to whom you have an obligation, and my opinion has been given above, as was yours before that.