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What if she mailed it to herself through the post, and her housemate accidentally opened the mail?

Then they are both innocent of their 'crimes' as they were both unintentional.

She wouldn't have intended her housemate to open it, so it wasn't a threat.

And her housemate didn't intend to open it, so it wasn't Obstruction of Correspondence. https://www.law.cornell.edu/uscode/text/18/1702