Postfix email delivery fails with `error: postdrop: warning: mail_queue_enter: create file maildrop/xxxxxx.xxxxx: Permission denied`

Issue

• Email delivery fails with following error in /var/log/maillog

  error: postdrop: warning: mail_queue_enter: create file maildrop/xxxxxx.xxxxx: Permission denied
  

• System has a large number of sendmail/postdrop/CROND processes:

  $ cat ps |awk '{ if ($8 ~ "S*") { print $0 }}' | grep sendmail | wc -l
  3578
  
  $ cat ps |awk '{ if ($8 ~ "S*") { print $0 }}' | grep sendmail | head -n 1
  user      307  0.0  0.0  81332  3348 ?        S    Dec26   0:00 /usr/sbin/sendmail -FCronDaemon -i -odi -oem -oi -t -f root
  
  $ cat ps |awk '{ if ($8 ~ "S*") { print $0 }}' | grep postdrop | wc -l
  3578
  
  $ cat ps |awk '{ if ($8 ~ "S*") { print $0 }}' | grep postdrop | head -n 1
  user      315  0.0  0.0  81296  3364 ?        S    Dec26   0:00 /usr/sbin/postdrop -r
  
  $ cat ps |awk '{ if ($8 ~ "S*") { print $0 }}' | grep CROND | wc -l
  3578