Self-introduction in Letter

[leeshee]

Hello everybody, I wish to know it is okay to begin with "This is XXX from XXX" as a self-introduction when writing to someone for the first time, or if such an introduction is necessary considering the sender's information will be given below.

Thank you very much for your help!

[Raymott]

Hello everybody, I wish to know it is okay to begin with "This is XXX from YYY" as a self-introduction when writing to someone for the first time, or if such an introduction is necessary considering the sender's information will be given below.

Thank you very much for your help!

You don't need to do this. You will have signed the letter and have your name on it.
You can say:

Dear ZZZ,
I represent YYY. I'm writing to introduce myself.
...
Yours Sincerely,
XXX

By the way, if you going to use variables, such as XXX in your posts, you need to use different variables for different values - otherwise they all mean the same thing.