In order to ensure the security of users' mailboxes, QQ mailboxes are set with POP3/SMTP/IMAP switches. The default setting of the system is "Off". When users need these functions to send and receive e-mail using client software, please turn it on.
References:
Baidu Baijiahao-QQ mailbox port number QQ mailbox port number
Port number: In network technology, ports include logical ports and physical ports. Physical ports refer to physically existing ports, such as interfaces on ADSL modems, hubs, switches and routers for connecting other network devices, such as RJ-45 ports and SC ports. Logical port refers to the port used to distinguish services in a logical sense, such as the service port in TCP/IP protocol. Port numbers range from 0 to 65535, such as port 80 for browsing web services and port 2 1 for FTP services. Due to the large number of physical ports and logical ports, in order to distinguish the ports, each port is numbered, which is the port number.