Any mail, other than the remittance of an amount withheld or deducted (most often, a source deduction), or a payment by a corporation, sent by first class mail or its equivalent, is deemed to be received by CRA the day it was sent (Subsection 248(7)).
Though Canada Post no longer provides first class mail, CRA has acknowledged that Canada Post “letter mail” would be considered equivalent. In addition, CRA has acknowledged in the same Roundtable Question and IT-433R, Paragraph 4, that an “item entrusted to a courier service for prompt delivery is considered equivalent to first class mail”.
For further information see VTN Monthly Tax Update Seminar, Issue No. 420