clear postfix mail queue

for f in ` mailq |grep .com -B 2 |grep ^[A-Z0-9]|awk '{print $1}'` ; do postsuper -d $f; done

Also, fun times here:
http://www.postfix.org/docs.html

clearing a mail queue

#sendmail
sendmail -qS -v subdomain.test.com

#postfix
for f in ` mailq |egrep subdomain.test.com -B 2 |grep ^[A-Z0-9]|awk '{print $1}'` ; do postsuper -d $f; done