What is the test for proving receipt of a letter?

It is "well settled" law that "proof that a letter properly directed was placed in a post office creates a presumption that it reached its destination in usual time and was actually received by the person to whom it was addressed." Hagner v. United States, 285 U.S. 427, 430 (1932). This presumption of receipt is "very strong" and "can only be rebutted by specific facts and not by invoking another presumption and not by a mere affidavit to the contrary . . . . Evidence of an objective nature going beyond the claimant's statement of non-receipt is

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