You are a landlord so I am assuming that you do not reside at the scheme.
Is the physical mail box that you are referring to a physical mail box at the scheme or is it at your actual place of residence?
If the mail box is at the scheme and you do not reside there then why is the Strata Manager sending Notices to that address?
Check with the Strata Manager to find out what address they have on the Strata Roll for the sending of correspondence and Notices to you. Update the Roll if necessary.
If the SM has made a mistake and sent the Notice to an incorrect address then you should not have to pay for their mistake.