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Ping and traceroute are the go-to tools for diagnosing routing issues. Ping is a very straightforward tool. It transmits a "echo request" packet over the Internet Control Message Protocol (ICMP) to the destination device, which responds with a "echo response" packet.
If your ISP provides a catch-all POP3 mailbox and someone sends a message to a non-existent user, or if someone sends a message to you with a BCC, the recipient email address will not be in the headers anyplace that VPOP3 can detect it, routing issues can arise.
Ping one computer using another computer on the same network to see if your router is working. If your router is in good working order, you should be able to do so. The firewall on your machine must also be turned off.
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