Here are some corrected ways to phrase it:
* "I am sending you this letter with reference to..." (This is the most common and grammatically correct way to phrase it.)
* "This letter is in reference to..."
* "Regarding..." (followed by the subject of the letter)
* "With regard to..." (followed by the subject of the letter)
For example, you could say:
* "I am sending you this letter with reference to your recent inquiry."
* "This letter is in reference to your order number 12345."
* "Regarding your recent order, I am writing to..."
* "With regard to your request for information, I am pleased to provide..."
Remember to choose the phrasing that best suits the tone and context of your letter.