Postfix can't be changed "maildrop" to "dovecot"

[truongd...@gmail.com]

Hi,
I have tried config postfix with dovecot, roundcube on virtual domain. I got these warning while sending an email.

Aug 5 11:03:51 cmail postfix/smtpd[14040]: disconnect from cmail.server.com[192.168.11.100] ehlo=1 mail=1 rcpt=1 data=1 quit=1 commands=5
Aug 5 11:04:55 cmail postfix/postfix-script[14068]: refreshing the Postfix mail system
Aug 5 11:04:55 cmail postfix/master[1605]: reload -- version 3.1.1, configuration /etc/postfix
Aug 5 11:04:55 cmail postfix/oqmgr[14073]: 636C3303F: from=<staff01@domain01>, size=525, nrcpt=1 (queue active)
Aug 5 11:04:55 cmail postfix/oqmgr[14073]: warning: connect to transport private/maildrop: Connection refused
Aug 5 11:04:55 cmail postfix/error[14075]: 636C3303F: to=<staff01@domain01>, relay=none, delay=375, delays=375/0.02/0/0.01, dsn=4.3.0, status=deferred (mail transport unavailable)
Aug 5 11:05:00 cmail dovecot: master: Warning: SIGHUP received - reloading configuration

I have set "virtual_transport = dovecot" in main.cf and add the following lines in master.conf, also comment "maildrop" lines in master.conf.

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

#maildrop unix - n n - - pipe
# flags=DRhu user=vmail argv=/var/spool/postfix/private/maildrop -d ${recipient}

But these warnings stills appears. I don't know how to switch from maildrop to dovecot. I really appreciate any help. Thanks !