Should I capitalize the starting sentence after a greeting that ends in a comma (e.g. "Hello,")?

Should I open the starting sentence after comma in adressing "Hi," ("Hello,") with capital letter?

Hi, Xxxl, Dear Xxx L,
let me ...

vs.

Dear Xxx L,
Let me ...

In Russian it is not though it is more than frequently being goofed.

Related question:

• Comma place: “Hello Mr. Black,…” vs “Hello, Mr. Black…”

I did a quick Google survey (ie searching for 'letters from the queen' etc in Google images and looking at the scanned letters that are generally the result).

-The Queen seems to use a comma and capital

Dear Mr Subject,

Thank you for...

-The President seems to use a colon and capital

Dear Mr Fellow American:

Thank you for..

-The Pope generally uses a comma but NO capital

Dear Mr Believer,

thank you for...

An observation rather than an answer, but if you wanted a precedent...

Since this is an informal and nonstandard mode of address (as compared to the more usual "Dear X,") I would say that rules of style are less important than for more conventional letters.

However, as far as I can tell, there is no good reason to oppose conventional style in this case. I would therefore recommend that you stick with the tried-and-true style of using a capital letter to start the letter proper:

Hi Xxxl,

Let me enquire into the origin of your weird name...

Note that there should be no comma between "Hi" and the name of the addressee.

Formal letters often seem to separate salutation and first line with a colon and thus continuing with a capital letter seems fit:

Dear Ms. Last:

Let me ...