What is a Post Office Box?
A post office box, often called a PO or P.O. box is a box located in a central post office where you can opt to receive all or most of your mail. The post office box is normally rented for a small fee on a monthly or yearly basis, and customers can choose to have their mail routed to their post office box, or merely give their post office box number as their address. In some countries, having a post office box is the only means by which you can receive mail, because delivery of mail may not cover all outlying or rural areas.
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In simple words, If I have to receive mail, i need to acquire a PO Box. Then all the mails for me will be delivered there, and to collect those i have to go to Post Office to check out my PO Box.