It may appear to be a touch archaic, but it is more official to use "I am writing to." However, it is still used in commercial correspondence. It is clear that the author and receiver do not have a close relationship. Therefore, there is no need for formalities. An alternative form is "You are about to receive an email from me." This appears more modern and avoids any ambiguity regarding the relationship between the two parties. As long as you are writing to someone who you believe to be your friend or colleague, then you can use either version of the sentence.