One of the most basic, and useful, tools for troubleshooting email systems is a Telnet client.
Unfortunately, since Windows XP, Microsoft do not install the Telnet client into Windows by default, so you may need to install the Telnet client yourself.
To do this, go to Control Panel → Programs and Features.
Choose Turn Windows features on or off in the left pane of the window
Scroll down until you find Telnet Client (note do NOT install “Telnet Server”, that is a security risk, unlike the Telnet Client). Check the box next to Telnet Client and press OK
You will normally not need the original installation disks to do this
To use the Telnet client, go to Start → Run in Windows, and type cmd and press OK
In the command prompt window that appears, type:
Telnet <address of VPOP3 computer> <port number>
For instance, to test the POP3 service (port 110) if VPOP3 is on 192.168.1.1, type:
Telnet 192.168.1.1 110
The response should say something like:
+OK VPOP3 Server Ready <random.text>
If this is the response you get then you can type
USER <username>
followed by
PASS <password>
eg
+OK VPOP3 Server Ready <6c.16a8.4069e3ac> USER test +OK User Accepted, PASSword required PASS testpass +OK test has 23 message(s) (121552 octets) QUIT +OK VPOP3 Server Closing Connection
That shows a valid login attempt. Any errors received during this process will be useful diagnostic information. This is especially helpful if you are limited to using an email client such as Microsoft Outlook which usually hides errors during the logon process so you can't see the error messages generated by the server.
Note that while typing commands in this way, you cannot use the backspace or other editing keys, you must type accurately the first time. If you make a mistake it is best to start again.
SMTP is usually on port 25, and the response should say:
220 <host name> VPOP3 ESMTP Server Ready
IMAP4 requires VPOP3 Enterprise. The usual port for IMAP4 is port 143, and the response should say:
* OK VPOP3 IMAP4rev1 Server
If the VPOP3 name is not there, then this may indicate that you have another mail server on the same computer as VPOP3, which is “taking over”. If the response includes error text, then you can search for that text here on the Wiki, or it will be useful information to give when you contact technical support for help.