www-data  4650  0.0  0.1  43428  4532 ?        S    04:58   0:00 /usr/sbin/postdrop -r
www-data  4855  0.0  0.1  43428  4528 ?        S    05:02   0:00 /usr/sbin/postdrop -r
www-data  6940  0.0  0.1  43428  4528 ?        S    05:57   0:00 /usr/sbin/postdrop -r
www-data  7251  0.0  0.1  43428  4528 ?        S    Nov09   0:00 /usr/sbin/postdrop -r
www-data 14872  0.0  0.1  43428  4532 ?        S    08:52   0:00 /usr/sbin/postdrop -r

My challenge:

I see a lot of error in apache error.log

postdrop: warning: mail_queue_enter: create file maildrop/733089.31799: Permission denied
postdrop: warning: mail_queue_enter: create file maildrop/240934.11141: Permission denied

Let login as www-data:

su -s /bin/bash /www

Run it as www-data

/usr/sbin/postdrop
postdrop: warning: mail_queue_enter: create file maildrop/338489.16902: Permission denied

Solution:
chmod -R 777 /var/spool/postfix/maildrop/