In Montana, landlords must provide 24 hours notice unless it is a case of emergency.

Source: 70-24-312

No. The tenant cannot unreasonably deny access to the landlord in order to inspect the premises, make repairs or improvements, supply services, or show the dwelling to prospective tenants, purchasers, workers, contractors, etc.

However, the landlord cannot abuse the right of access to harass the tenant. Except in emergencies or unless it is impracticable to do so, the landlord must give the tenant twenty-four hours notice (verbal or written) of her/his intent to enter the premises. The landlord can enter only at reasonable times. Notice may be considered impractical if the tenant is absent from the dwelling for an extended period. 70-24-312 70-24-322

Source: 70-24-312 and 70-24-322

Anyone intentionally opening your mail is a Federal crime under statute; however, your landlord opening your mail on accident is not a crime. If they open it up once, you don’t have much standing. If they do it repeatedly it’s probably worth reporting. As always, talk to your landlord and document all communication before pursuing legal action.