This error means that VPOP3 has detected that there is not enough free memory available on the server. If this happens it refuses to accept new messages because that may make the PC crash and possibly lose data.

You need to check whether there is, in fact, enough RAM installed on the VPOP3 computer. If there isn't it will run very slowly, so it may be worth installing more RAM into the server PC.

In the meantime you can solve the problem by increasing the amount of virtual memory available to Windows on the VPOP3 computer - see http://windows.microsoft.com/en-us/windows/change-virtual-memory-size#1TC=windows-7 for instructions how to do this on Windows 7, other versions of Windows are similar.