This error has two common causes:
A) Receive connector conflict
If you have multiple receive connectors configured, there may be a conflict. Remove any duplicate or conflicting connectors and ensure the "Default Frontend [SERVERNAME]" connector is correctly configured with Anonymous Users permitted.
B) TLS / DNS / network issues
- Disable TLS on the receive connector (POPcon uses plain SMTP, not STARTTLS).
- Disable any unused network adapters on the Exchange server.
- Configure the server to use IPv4 only.
- Verify DNS entries are correct and that the server clock is synchronized (use NTP).