postfix/procmail

[Henry Harvey]

I'm sorry I am a newbie on this.
I am actually running procmail 

So in my /etc/postfix.in/main.cf
I have:
mailbox_command = /usr/bin/procmail -Y -a $DOMAIN

and added:
defer_transports = smtp local virtual relay

Is this right for postfix installations?
can somebody explain to me the defer_transports?
Thanks

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