With regard to

It is pointed out specifically but without any explanation by an English learning website that “with regard to” can not be used at the beginning of the first paragraph in a business letter like: With regard to your letter dated …, I am writing to …?

I know it is ok to be used in the middle of a sentence but I am really confused by what the website says.

Thank you for your help!

Hi Millie,

It doesn’t have to be as strict as that and the use of this expression is all right at the beginning of the first paragraph.