There are several synonyms for the term "PO Box No." One of the most commonly used synonyms is "post office box number," which is often abbreviated as "P.O. Box." Other synonyms include "mail drop" and "locking mailbox." These synonyms all refer to a specific address that is used for receiving mail, but they may have different connotations depending on their usage. For example, a "mail drop" may refer to a mailbox that is used for anonymous purposes, while a "locking mailbox" may refer to a mailbox that is used for secure delivery of packages and important documents. Regardless of the synonym used, a PO Box No is a convenient way to receive mail and keep it separate from a home or business address.