This error does not come from Exchange — it comes from another mail server (typically hMailServer or a similar SMTP server) that is running on the same machine as POPcon and is listening on port 25.
POPcon connects to whatever answers on port 25 at the configured address, and if that is hMailServer or another authenticated mail server, it will receive this rejection.
Fix options
- Remove the conflicting mail server if it is not needed.
- Change the port — move the conflicting server to a different port, and configure POPcon's Exchange delivery port to match (set on the EXCHANGE configuration tab in POPcon).