you can also try qmHandle from the command line. It may work faster depending on the number of emails stuck in the remote:
man qmHandle (to see a list of easy commands)
#to clear qmail queue
#stop qmail
/etc/init.d/smtpfront-qmail stop
/etc/init.d/qmail stop
qmHandle -D
#restart qmail
/etc/init.d/qmail start
/etc/init.d/smtpfront-qmail start