get mails only when requested with fetchmail/postfix
 

I want to use fetchmail and postfix. Is it possible to start fetchmail only when a mail client asks postfix for new mails?

The background is that I do not want postfix to store mails for certain mail accounts, for some users fetch their mails over different channels.

When these useres want to fetch their mails from my postfix, fetchmail should get them, deliver them to postfix, and all the mails should be delivered to the user's mail client.