master.cf :

dovecot   unix  -       n       n       -       -       pipe
    flags=DRhu user=vmail:vmail argv=/usr/lib/dovecot/deliver -d ${recipient}

main.cf:

virtual_transport = dovecot

mail.log:
Oct  5 21:36:02 host postfix/qmgr[9717]: warning: connect to transport dovecot: No such file or directory