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.