Here’s some handy tips for managing your qmail queue on a linux machine, which can be helpful for debugging:

Clear your Queue

Firstly, stop Qmail from your PLESK control panel in Server > Services, or run this command:
service qmail stop

Now log into a shell terminal (probably via SSH) and then run these commands:

find /var/qmail/queue/mess -type f -exec rm {} \;
find /var/qmail/queue/info -type f -exec rm {} \;
find /var/qmail/queue/local -type f -exec rm {} \;
find /var/qmail/queue/intd -type f -exec rm {} \;
find /var/qmail/queue/todo -type f -exec rm {} \;
find /var/qmail/queue/remote -type f -exec rm {} \;

Once those are finished, just restart your mail server again from PLESK and you’re done! Alternatively run this
service qmail start

View how many messages are in your queue:

Run this command:

/var/qmail/bin/qmail-qstat

And you’ll get something along the lines of:

messages in queue: xxxxxx
messages in queue but not yet preprocessed: x