You've asked a good question about a grammatical structure that we don't often see. In your sentence "am writing" is the present continuous verb and it is followed by a preposition that ends in -ing. The verb can be in written in any of the continuous aspects/tenses and followed by a preposition with an -ing ending. Here is a short list of prepositions with an -ing ending: assuming barring concerning considering during following including involving pending regarding succeeding These prepositions are difficult to recognize as prepositions, but they are, and they are usually followed by a noun. Here are a few examples of a continuous verb followed by one of the -ing prepositions: - He was sleeping during the movie. - They were hesitating assuming the worst. - You will be going barring any interruptions. - The company has been downsizing including laying off workers. - I had been reading during the flight. - We will be waiting pending the president's decision. As you can see, these structures, i.e., continuous verbs followed by an -ing preposition are not commonly used structures.

It's fine. You could also say "I am writing in regard to..."

thank you!

I think so