Порт 110 (POP3)

TCP port 110 is the standard port for the Post Office Protocol version 3 (POP3). POP3 is an application-layer internet standard protocol used by email clients to retrieve email from a mail server. It operates by allowing a client to connect to a server, authenticate, and then download messages. After downloading, the client typically deletes the messages from the server (though this behavior is configurable). POP3 is a relatively simple protocol designed for offline email access, where users download messages and then disconnect from the server to read and manage them. It's a store-and-forward service, distinct from protocols like IMAP which are designed for online, synchronized access.

The protocol works via a series of commands sent from the client to the server. Common commands include USER (specifying the username), PASS (providing the password), LIST (listing the messages on the server), RETR (retrieving a specific message), DELE (marking a message for deletion), RSET (unmarking messages marked for deletion), NOOP (no operation), and QUIT (terminating the session). The server responds to each command with a status code, typically either +OK for success or -ERR for failure. Because POP3 was originally designed without encryption, it's vulnerable to eavesdropping, making the plaintext transmission of usernames and passwords a significant security risk. Secure versions of POP3, such as POP3S (using SSL/TLS on port 995), were introduced to mitigate these risks.

## Firewall Recommendations

Blocking port 110 is strongly recommended if you're not actively using POP3 and can migrate to a more secure protocol like IMAP (port 143 or 993). If you must use POP3, prioritize using POP3S (port 995) with SSL/TLS encryption. Configure your email client to use the secure version and ensure the server supports and enforces it. If you need to allow access to port 110, restrict access to only trusted networks or IP addresses. Implement strong authentication policies, such as requiring strong passwords and enabling multi-factor authentication where possible. Regularly monitor network traffic for suspicious activity and keep your email server software up to date with the latest security patches to mitigate potential vulnerabilities. Consider implementing intrusion detection and prevention systems (IDS/IPS) to detect and block malicious traffic targeting port 110.