Sometimes, you have dozens of messages clogging your Postfix queue, all with the same bogus recipient.

Here is how to delete them in a go, using the postsuper command and some scripting magic (of course, you need to replace address@example.com with the desired recipient):

mailq | tail -n +2 | grep -v '^ *(' | awk  'BEGIN { RS = "" }
              # $7=sender, $8=recipient1, $9=recipient2
              { if ($8 == "address@example.com" && $9 == "")
                    print $1 }
          ' | tr -d '*!' | sudo postsuper -d -

(Adapted from the postsuper man page.)