When is service by mail considered legally received in Ontario?

Where Ontario's civil rules allow a document to be served by mail, the law generally treats it as received on a specific deemed date after mailing, rather than requiring proof the recipient actually opened the envelope. This "deemed service" approach exists because it would otherwise be almost impossible to prove exactly when, or whether, someone personally read something sent by ordinary mail. The deemed date is usually calculated by counting forward a set number of days from when the document was mailed, and that deemed date is what starts any response deadline running, regardless of when the documents were actually seen.

Not every document in a lawsuit can be served by mail in the first place; some steps, particularly the original statement of claim, generally require a more direct method of service unless the parties or a court order have specifically allowed mail. Where mail service is proper, a party who claims they never actually received anything faces a real evidentiary hurdle in challenging the deemed date, though it isn't automatically impossible if there's genuine evidence the mail never reached them. Keeping a mailing address current with the court and the other side reduces the risk of missing something served this way.