How to solve [Postfix] – warning: mail_queue_enter: create file maildrop Permission denied

$ sendmail
postdrop: warning: mail_queue_enter: create file maildrop/191857.27042: Permission denied

$ sudo postfix check

postfix/postfix-script: warning: not owned by group postdrop: /var/spool/postfix/maildrop

$ sudo ls -ld /var/spool/postfix/maildrop

drwx-wx---. 2 postfix postfix 4096 Sep 19 15:09 /var/spool/postfix/maildrop

[i88ca@stg ~]$ sudo /etc/init.d/postfix stop

Shutting down postfix:                                     [  OK  ]

[i88ca@stg ~]$ sudo killall -9 postdrop

[i88ca@stg ~]$ sudo chgrp -R postdrop /var/spool/postfix/maildrop/

[i88ca@stg ~]$ sudo ls -ld /var/spool/postfix/maildrop

drwx-wx---. 2 postfix postdrop 4096 Sep 19 15:50 /var/spool/postfix/maildrop

[i88ca@stg ~]$ sudo postfix check

[i88ca@stg ~]$ sudo /etc/init.d/postfix start

Starting postfix:                                          [  OK  ]

[i88ca@stg ~]$ sendmail