1. The address of the sending and receiving mail server is set incorrectly, or the IP address of the server cannot be resolved in the network environment. It is recommended that you enter the Ping mail server address -t when Windows starts running, and enter the ping IP address in the corresponding setting column of the client software.
2. The security level of firewall and mail detection program is too high, which directly interrupts data transmission. You can adjust the security level setting or temporarily close such programs according to actual needs.
3. The server host or router of the local area network (corporate intranet) is abnormally connected to the external network. I suggest you contact the system administrator to check.
4. Set "This server requires secure connection (SSL)".