Yikes. At first glance, it appears that the author is trying to show the difference between an apple and an apple. The "overcoming of the presumption" and the "plaintiff's burden" are one in the same, no?
It means the defendant won, but the court didn't say why exactly. Either the plaintiff didn't meet his burden re: the mailbox rule, OR the plaintiff met his burden, but the defendant produced enough evidence to overcome the presumption of receipt.