Postfix is a mail server that started life at IBM research as an alternative to the widely-used Sendmail program. Newer versions of Plesk install with Postfix as the default mail server but when upgrading older versions of Plesk, Qmail may remain the default mail server. It is suggested to use Postfix, since postfix is an actively maintained mail server package widely used in production environments
# View emails in the queue
# read email from mail queue (header and body)
postcat -q MAIL_ID
# number of emails in Mail queue
postqueue -p | grep -c "^[A-Z0-9]"
# Flush the queue: attempt to deliver all queued mail.
To remove all mail from the queue :
# postsuper -d ALL
To remove all mails in the deferred queue :
# postsuper -d ALL deferred