If you get an error while sending a message using Webmail, such as
“Error sending message after authentication - error 0 - text 502 Error: command not implemented”
this means that you have another SMTP server running on the VPOP3 computer, and when the VPOP3 Webmail service tries to send the message it is sending it to this other SMTP server, rather than to the VPOP3 server. The Webmail service uses internal commands which the other SMTP server won't recognise - hence this error message.
Note that the exact text of the error message may vary, as it is generated by the third party SMTP server, not by VPOP3 - the Error sending message after authentication section will be the same though.
To find out what the other SMTP server is, go to a command prompt on the VPOP3 computer and run:
telnet 127.0.0.1 25
The response you get back may show which SMTP server program is running (often the Microsoft SMTP server). You need to stop the other SMTP server from running, or move the other SMTP service or the VPOP3 SMTP service to use a different TCP/IP port.
n.b. If you are running Windows Vista/2008 or later, then the Telnet client may not be installed by default - see this Microsoft Technet article for instructions on installing it.