Which port should be opened in the firewalls to allow POP3 mail delivery?

The correct port to open for Post Office Protocol version 3 (POP3) mail delivery is 110. POP3 is used by email clients to retrieve emails from a mail server, allowing users to download and manage their messages locally. By opening port 110 in the firewalls, you ensure that POP3 traffic can flow freely, enabling email clients to connect to mail servers and retrieve emails as needed.

The other port numbers listed serve different purposes: port 101 is typically associated with the "POP3" alternative protocol, which is not widely used; port 123 is used for the Network Time Protocol (NTP), responsible for synchronizing clocks over a network; and port 161 is designated for the Simple Network Management Protocol (SNMP), which is used for management and monitoring of network devices. None of these ports would facilitate POP3 mail delivery, making 110 the necessary choice for this service.