If you're a Shopify Plus merchant and don't want your customers to place orders containing PO Boxes, you may want to enable the address rule and change the severity to "Strict"
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App Dashboard > Validation Rules > Address Rules > Do the following when a shipping address violates one of the rules: > Block
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Please note that PO Box orders can slip through the app via express payment options, Address Validator won't be able to block these orders but will prompt customers for an alternative address on the Thank You page.
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Also, you can enable email notifications in case PO Box orders slip through:

​App Dashboard > Email Notifications > Other Notifications > Emails when customers proceed with PO Box addresses