As for the letter from the insurance company, here's what the province says about deemed date of service:
Deemed date of service
(2) Notice sent by registered mail under clause (1) (a) or by mail under clause (1) (b) shall be deemed to have been given on the seventh day after the mailing unless the person to whom the notice is sent establishes that he or she did not, acting in good faith, through absence, accident, illness or other cause beyond his or her control, receive the notice. 2000, c. 26, Sched. O, s. 4.
Here's a case of due dilligence, not from Ontario and from which the previous quote doesnt apply, but you'll understand a lot more about due dilligence. Check all related decisions also by clicking "search for decisions citing this decision" at the top:
R. v. Miller, 2006 NBPC 2 (CanLII)
Basically, the related decisions are here (they will give you a lot of insight on due diligence):
Hill v. R., 1973 CanLII 36 (SCC)
R. v. Hundal, 1993 CanLII 120 (SCC)
R. v. Lowe, 1991 CanLII 2557 (NS CA)
R. v. MacDougall, 1982 CanLII 212 (SCC)
R. v. Pierce Fisheries Ltd., 1970 CanLII 178 (SCC)
R. v. Sault Ste. Marie, 1978 CanLII 11 (SCC)
R. v. Tutton, 1989 CanLII 103 (SCC)
R. v. Wholesale Travel Group Inc., 1991 CanLII 39 (SCC)