If you get an error like:

-ERR VPOP3 Server access not allowed!

when trying to access VPOP3, then this means that the relevant server's IP Access Restrictions defined in VPOP3 do not allow access from your client computer.

To change the IP Access Restrictions, go to the Services page in the VPOP3 settings and click on the relevant service. Then go to the IP Access Restrictions tab.

In the access restrictions you should specify which networks have access to your computer. The Default for your LAN button will set up the access restrictions to sensible settings for your local network, but you may need to manually change them if you want to allow access from outside your local network.

See VPOP3 Access Security for more information.

Note that you must be very careful with allowing unauthenticated access to the SMTP service, as that can lead to VPOP3 becoming an open relay, which is a bad thing. See Allowing Mail Relay Through VPOP3 for more details.

In these versions, the access restrictions are specified as text. Each line is a network address and subnet mask, eg:

 192.168.0.0 255.255.255.0

means the subnet 192.168.0.0-192.168.0.255

In VPOP3 2.0.0 and later, it will configure itself with automatically detected access restrictions at installation. This is usually correct, but may not be if you have several local networks connected together.

In VPOP3 1.5.0 and earlier go to the Local Servers tab and press the Configure button on the relevant line). In the Window that appears, there is a text box called Access Restrictions

Very early versions of VPOP3 did not have any IP access restrictions capability because they were developed when it was less of an issue. VPOP3 1.4.3 to 1.5.0 will ask at installation if you want it to configure a default set of access restrictions, with a message box saying:

Do you want VPOP3 to install Default Access Restrictions for the local services. This restricts access to the VPOP3 services so that only the following address ranges can access VPOP3:
- 10.x.y.z
- 127.x.y.z
- 172.16.x.y
- 192.168.x.y

If your local network has a different network range, do not install the default access restrictions, but you should configure them manually on the Local Servers → Config windows

If you press Yes in response to this prompt, then access restrictions allowing the above address ranges will be installed, and other addresses will not be able to access the VPOP3 server, so if you pressed Yes when you are actually using a different network address range, then you may have inadvertently prevented access to VPOP3 from your local network.