Yes this is legal. The fact is that as a general rule an employee has no expectation of privacy in the workplace. And in fact postal regulations say that mail delivered to a business, even if addressed to a specific person, is delivered to the business itself, and therefore it can decide how to distribute it from there. Accordingly, it would behoove an employee to keep their personal mail out of their place of work. Otherwise, they risk a package, letter, etc. being opened by co-workers.Note: An employer has the same right to monitor your email on a company email system/server, or your website viewing over company internet.