Nerdy cousin to the truly weird Crouching Tiger, Hidden Canada (CTHC) trope, Blue Mailbox Syndrome was spawned in the 1980's by the same confluence of over-the-top Hollywood production costs, a very low Canadian dollar and Canadian government subsidies for domestic TV and movie production. American productions flocked to Toronto, Vancouver and (to a lesser degree) Montreal.
Unlike CTHC, Blue Mailbox Syndrome shows are essentially American productions totally offshored to a Canadian location. The filmmakers, technicians and many of the secondary character actors will be Canadian for domestic work-quota reasons, but the producers, directors, writers and lead actors will almost always be American. CTHC productions, by contrast, are usually conceived by a Canadian-based production company and later sold to an American broadcaster.
The Blue Mailbox Syndrome trope is defined by the often laughable cosmetic steps used to disguise Canadian locations as American, easily symbolized by the replacement of red Canada Post mailboxes in street scenes with blue U.S. Postal Service ones. This trope often leads to serious or silly logical dissonance for Canadian viewers. The willing suspension of disbelief is hard to maintain when the blue USPS mailbox (Marine recruiting station, FBI office, governor's mansion, etc. etc.) is sitting next to a famous Canadian landmark, or even a Shopper's Drug Mart or Canadian Tire store.
Well-known Blue Mailbox Syndrome TV shows include X-Files, CSI (and all spinoffs), Stargate SG-1 (and all spinoffs) Mutant X, the 4400 and several iterations of The Twilight Zone and The Outer Limits.
Obviously, this trope doesn't apply to shows with futuristic or fantasy settings, like Battlestar Galactica or Andromeda, even if they were shot in Canada.
Well-known movies that fit this trope are too numerous to count, but one of the best examples is the X-Men franchise, all shot in Toronto, with the famous local landmark the Casa Loma standing in for the X-Mansion.
For a full listing of U.S. shows shot in Vancouver, see Stargate City.