DefaultPort is a Published property in TIdPOP3Server that indicates the default port number used in listeneing threads that accept new client connections to the server. 

DefaultPort is assigned to the Bindings used for the server. 

Change the value in DefaultPort prior to creating Bindings (using the IDE) or setting the server's Active property to True. Changing the value in DefaultPort after the server has created listener threads and their Bindings has no effect. 

The initial value for DefaultPort is IdPORT_POP3, as assigned during initialization of the component.

TIdTCPServer.Bindings Active