"Mail transfer to Exchange failed: Connection to host denied"

Exchange's SMTP service is not responding on port 25. Common causes:

• Antivirus blocking port 25 — some endpoint security products intercept SMTP. Try disabling temporarily.
• Port changed — another application may have claimed port 25.
• POPcon lacks connection permission — verify the receive connector allows the POPcon IP address.
• Antispam rejecting connection — add the POPcon server's IP to the Exchange IP Allow List.

Test with TELNET

telnet exchangeservername 25

You should see an SMTP banner (220 ...). If the connection is refused or times out, port 25 is blocked between the two machines.

Screenshots — step by step