The Real Estate Settlement Procedures Act (RESPA) condified at 12 U.S.C. 2601 et seq. is the law that applies to servicing transfer notifications. The owner of the loan need not be the servicer of the loan and vis-a-versa. Therefore, it is possible that the loan was sold effective as of April 16, but that the new servicer wasn't appointed until April 21. To determine who the proper owner is, I recommend contacting the new servicer. Good luck.