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POST Office Protocol 3 (POP3) is the standard protocol used by email clients to retrieve emails from a mail server. By default, POP3 operates on port 110. This port is designated specifically for this protocol, allowing email clients to connect to the server to fetch emails for users.

Understanding the purpose of this protocol is crucial. POP3 allows users to download emails to their local system, which can then be accessed without an internet connection. Once the emails are downloaded, they are typically deleted from the server, although some configurations allow for emails to remain on the server.

The other ports listed serve different functions:

- Port 109 is used for the older version of the POP protocol (POP2), which is not widely used today.

- Port 115 is designated for Simple File Transfer Protocol (SFTP), a method of transferring files rather than handling emails.

- Port 123 is utilized for the Network Time Protocol (NTP), which is unrelated to email services.

Thus, the correct association of POP3 with port 110 is foundational for understanding email retrieval processes in computer networking.