Which protocol operates on port 110 for email retrieval?

The protocol that operates on port 110 for email retrieval is POP3, which stands for Post Office Protocol version 3. This protocol is designed specifically to allow email clients to retrieve emails from a mail server. When using POP3, emails are typically downloaded from the server and can be stored locally on the user's device, thereby enabling offline access to the messages.

Understanding the purpose of POP3 helps in recognizing how it differs from other email protocols. For instance, IMAP (Internet Message Access Protocol) uses port 143 and allows users to manage their emails directly on the mail server, providing a different method of interaction that maintains email synchronization across multiple devices. SMTP (Simple Mail Transfer Protocol), which operates on port 25 and occasionally on port 587, is used for sending emails rather than retrieving them. Furthermore, DNS (Domain Name System) is unrelated to email retrieval, as it resolves domain names to IP addresses and operates on port 53.

The specification of POP3 using port 110 is recognized in networking standards, making it clear why this protocol is the correct choice for the question about email retrieval.