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If you are referring to a search warrant, then they could open the mail only if it was encompassed within their warrant. That would depend on the particular wording of the warrant. They can generally open your mail, the same way that they can look through your drawers and cabinets and search through your car, if they have a warrant, supported by probable cause. (Interestingly, if registered mail could not be opened by the police, the a person could mail his pot to himself, and virtually insulate himself from prosecution for possessing that pot.)
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