I'm trying to identify a story which appeared in an early issue of Isaac Asimov's Science Fiction Magazine (so, a year or so either side of 1980). It takes the form of a series of story submission cover letters (sent to IASFM itself) and their associated rejection slips. The in-universe author claims that the story has actually been written by an alien who is living with her; the magazine editor clearly doesn't believe this but humours her. Each time he rejects the story he gives her advice; the story is revised but to no avail. Finally the author advises the editor that the alien has left Earth in disgust but not before making a change to the space-time continuum that will benefit everybody. The editor's response is to accept the story but express ignorance of anyone called 'Isaac Asimov'. It's signed by the editor of Arthur C. Clarke's Science Fiction Magazine.
This is "One Rejection Too Many" by Patricia Nurse. Her only published story, it apppeared in Asimov's, July-August 1978.
Dear Miss Morrison,
I am very confused by your letter. Who is Isaac Asimov? I have checked with several publishers and none of them has heard of Isaac Asimov's Science Fiction Magazine, although the address on the envelope was correct for this magazine.
However, I was very impressed with your story and will be pleased to accept it for our next issue. Seldom do we receive a story combining such virtues as a well-conceived plot, plenty of human interest, and a delightfully subtle brand of humor.
George H. Scithers,
Arthur C. Clarke’s Science Fiction Magazine
The story can be read at the Internet Archive (lighter weight than the PDF at Luminist).