The author of this forged letter must have spent quite some time editing it so it doesn't have any obvious grammer errors, but the lack of sophistication is not hard to spot.

Example:

"When plagiarism is detected in the United States, it can end the career of the person who did it."

Have you heard of the word "perpetrator," professor?

One other clue is that such a condescending harangue and gross generalization in this letter about the entire Beida and even China is highly inconsistent with the typical style found in most American professors, and perhaps especially those from schools such as Yale.

This is likely to be the work of a student at PKU and definitely not that of a Yale professor.

Fortunately this shouldn't be hard to prove or disprove.