This error means that there are too many connections to the PostgreSQL database server.

If only VPOP3 is using the PostgreSQL database, then you can reduce the number of connections that VPOP3 will make to the database at once. This is set in the VPOP3 settings at Settings → Database → Connection, “Max Connections”. The default value here is 50

Also, there's a registry setting at HKEY_LOCAL_MACHINE\Software\PaulSmith\VPOP3\Database called 'AbsMaxPostGreSQLProcesses'. The default of this is 80. If the number of connections has reached the normal 'Max Connections' limit, and VPOP3 can't get a new connection within a few seconds, it will still make a new connection until it reaches the 'AbsMaxPostGreSQLProcesses' limit.

If VPOP3 is sharing the PostgreSQL database with some other software, then you will need to calculate maximum usage across the different programs, and adjust their maximum connection limits accordingly.

In the VPOP3\pgsql\data directory, is a text file called 'postgresql.conf' which defines the configuration of the PostgreSQL database server.

In that file is a line like 'max_connections = 100'. This defines the maximum connections allowed to the database server. In Windows this should not be raised above about 150. (If you are actually running PostgreSQL on a Linux server, then this limit does not apply). After changing this you will need to restart the PostgreSQL and VPOP3 services in Windows.