How to clear the mailqueue.

Check the mailq:

# /usr/local/psa/admin/bin/mailqueuemng -s

View 5 of the items for a certain domain:

# /usr/local/psa/admin/bin/mailqueuemng -s | grep -C 5 user@domain.com

# /usr/local/psa/admin/bin/mailqueuemng -D"

or

# postsuper -d ALL

If you get a permission error on an older PLesk 9 instance

# /usr/sbin/postsuper -d ALL

To remove specific emails from a domain:

# postqueue -p | tail -n +2 | awk 'BEGIN { RS = "" } /@domain\.com/ { print $1 }' | tr -d '*!' | postsuper -d -

Or

# mailq | tail -n +2 | grep -v '^ *(' | awk  'BEGIN { RS = "" } { if ($8 == "USER@EXAMPLE.COM" && $9 == "") print $1 } ' | tr -d '*!' | postsuper -d -