English term (edited): comma after

You don't need a comma in "Hello Christian", though you need one after:

Hello Christian,...

You do need it in

Sincerely,

Christian

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Note added at 12 mins (2004-09-09 16:41:07 GMT)
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According to this site, you do need the comma in the first example:

3. After \"good morning\" or similar salutation or phrase:

* good morning , Mister Smith .
* how are you , Susi .


http://www.is.cs.cmu.edu/trl_conventions/punctuation.html

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Note added at 14 mins (2004-09-09 16:43:41 GMT)
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Here is another example, I guess technically, you DO need the comma:
I often receive e-mails that begin with the following greeting: \"Hi Rachel.\" Although this is certainly a friendly way to begin a letter, it violates one of the many comma rules: 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, use a pair of commas to set off the direct address. Note the placement of commas in the example sentences below:

Example 1: Ladies and gentlemen, may I have your attention please? (Direct address at beginning of sentence)
Example 2: It was a pleasure to meet you, Sir. (Direct address at end of sentence)
Example 3: Thank you, my fellow grammarians, for remembering to use correct English. (Direct address in middle of sentence)

So, you see, the salutation \"Hi Rachel\" should be \"Hi, Rachel.\" A comma is needed between Hi and Rachel because it is a direct address.
http://www.grammarerrors.com/punctuation.html