Looking on the internet deeply has found these results:

I am writing to you to tell you is the most popular phrase on the web.

I am writing to tell you 110,000 results on the web	I am writing to you to tell you 1427,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
in Jul 8, 2011 ... I am writing to tell you that I have accepted a call to return to Trinity Church, Copley Square in Boston as their Priest I would be interested in (obtaining/receiving) ... I received your address from ... and would like to ... I am writing to tell you about ... Thank you for your letter of. ... Introduction of Product/Service. I am writing to tell you about . . . (Our new product) is coming out next month. This product/service is designed to (help you) . I am writing to tell you about... Referring to previous contact. Thank you for your email regarding... Thank you for contacting us. In reply to your request. Charge. This decision, in ...	It doesn't look like this phrase is very popular! The search could not find any examples on the internet.

I am writing to tell you

110,000 results on the web

I am writing to you to tell you

1427,000 results on the web

More popular!

in
Jul 8, 2011 ... I am writing to tell you that I have accepted a call to return to Trinity Church, Copley Square in Boston as their Priest
I would be interested in (obtaining/receiving) ... I received your address from ... and would like to ... I am writing to tell you about ... Thank you for your letter of.
... Introduction of Product/Service. I am writing to tell you about . . . (Our new product) is coming out next month. This product/service is designed to (help you) .
I am writing to tell you about... Referring to previous contact. Thank you for your email regarding... Thank you for contacting us. In reply to your request.
Charge. This decision, in ...

It doesn't look like this phrase is very popular! The search could not find any examples on the internet.