[ILUG-BOM] postfix query

Hi Al I have a postfix server and have a query on its functioning : What is the difference between Local Delivery Agent ( LDA ) and virtual delivery agent ... Do these two daemons work simultaneously.In my main.cffile i have thne following statements #virtual_transport=virtual ....... does this men that the LDA is not running. ?? My second question is what is spawn then following is an extract i got from the net , but if some one can give me a an explination in a more understandable language

( spawn The spawn process creates non-Postfix processes on request. It listens on a TCP port, Unix domain socket, or FIFO connected to the standard input, output, and error streams. )

thanks