Mail theft is a crime under Canadian law.
As with any criminal situation, individuals who believe they are the victim of a crime should contact the police. Individuals are able to initiate the process to determine whether criminal charges should be laid, though the final decision on whether to charge someone rests with the government authority, typically a Justice of the Peace.
A possibly relevant section of the Criminal Code is s.356.
Theft from mail
356 (1) Everyone commits an offence who
(i) anything sent by post, after it is deposited at a post office and before it is delivered, or after it is delivered but before it is in the possession of the addressee or of a person who may reasonably be considered to be authorized by the addressee to receive mail,
(ii) a bag, sack or other container or covering in which mail is conveyed, whether or not it contains mail, or
(iii) a key suited to a lock adopted for use by the Canada Post Corporation;
(a.1) with intent to commit an offence under paragraph (a), makes, possesses or uses a copy of a key suited to a lock adopted for use by the Canada Post Corporation, or a key suited to obtaining access to a receptacle or device provided for the receipt of mail;
(b) has in their possession anything that they know has been used to commit an offence under paragraph (a) or (a.1) or anything in respect of which they know that such an offence has been committed; or
(c) fraudulently redirects, or causes to be redirected, anything sent by post.
Hope you find this information helpful.