All carriers should independently verify any LOAs received for account changes.
Documents received from third-parties, without independently verifying with the customer of record, using the carriers own records, are just junk papers.
Almost no carriers verify LOAs by contacting the customer of record. Worse, they call the phone number on the letterhead provide by the scammer for "verification."
The U.S. Postal Service used to let random people change mail forwarding orders, without verifying with the original and new addresses. As you can guess, there were lots of fake forwarding orders and criminal activity. After USPS begin verifying mail forwarding orders by sending a letter to the ORIGINAL address and NEW address, mail forwarding fraud declined. Not zero, but declined.