Looking for a particular short story consisting entirely of rejection letter correspondence

This is "Who's Cribbing?" by Jack Lewis. It appeared in Fifty Short Science Fiction Tales, so that's the most likely place you might have read it. It consists of fictional correspondence between Lewis himself and various science fiction magazine editors. Every story that Lewis submits turns out to have been previously published by an early twentieth-century author and engineer named Todd Thromberry. For example, one letter is

April 2, 1952

Mr. Jack Lewis
90-26 219 St.
Queens Village, N.Y.

Dear Mr. Lewis:

We are returning your manuscript "The Ninth Dimension." At first glance, I had figured it a story well worthy of publication. Why wouldn't I? So did the editors of Comic Tales back in 1934 when the story was first published.

As you no doubt know, it was the great Todd Thromberry who wrote the story you tried to pass off on us as an original. Let me give you a word of caution concerning the penalties resulting from plagiarism.

Sincerely,

Doyle P. Gates
Science Fiction Editor
Deep Space Magazine

Eventually, Lewis comes to the conclusion that Thromberry figured out how to spy through time, and passed off Lewis's fiction as his own. He collects his submission and rejection letters as a work unto itself (the story "Who's Cribbing?" itself) and submits it, but he is informed that, ironically enough, Thromberry had already used the idea.