If a user can't log in to collect or send mail, then the first place to look is the SECURITY.LOG file.
This can be accessed from the Settings -> Diagnostics page in the VPOP3 settings, or as a text file in the VPOP3 logging directory (usually the VPOP3\_logs folder).
You should be able to find failed login attempts in the SECURITY.LOG file. If the login attempt isn't there, then it is probable that something is blocking the request from reaching VPOP3 at all. This could be a hardware router/firewall, or Internet security software, or that you are using the wrong server address.
Also, the intrusion protection component of VPOP3 will block IP addresses from being able to log in if it detects consistent suspicious activity from an IP address (eg due to consistent failed login attempts). This can be checked in Settings -> Security -> Manage Block List.
If you have found the failed login attempt in the SECURITY.LOG file, then the text related to the failed login attempt should indicate what the problem is.
- Failed Logon for Fred in SMTP from 220.127.116.11 means that 'Fred' tried to logon to the VPOP3 SMTP server from the IP address 18.104.22.168, and the password was incorrect
- Unknown User for email@example.com in IMAP4 from 22.214.171.124 means that someone tried to log on as 'firstname.lastname@example.org'. VPOP3 user names are just a user name, not a full email address, so it is possible that the user was meant to log on as 'fred' rather than 'email@example.com'
- User Locked Out - fred in POP3 from 126.96.36.199 means that the user 'fred' has been locked out from VPOP3 from the listed IP address due to too many failed login attempts. You can unlock the user by going to Users in VPOP3, editing the user and removing the check from the 'Account Locked' box in the General tab.
- Access Restrictions block for joe in SMTP from 188.8.131.52 means that the IP Access Restrictions for the relevant VPOP3 Service do not allow that user to access VPOP3 from that IP address.
You can also try TELNETting to VPOP3 from the user's IP address using the relevant port number (eg 'telnet 192.168.1.1 25' to try to connect to VPOP3's default SMTP port). This will show if it is being blocked from connecting (eg due to firewall or Internet security software) or is connecting, but VPOP3 is giving an error message after connection.