how can i delete the send queued messages?

i've got answer at his mail. i don't know how many it was

I can not send mail from my account.
After activating the outgoing log file, was this error listed :

17/09-2014 09:59:12 SMTP IN :
501 Syntax error - line too long

17/09-2014 09:59:12 SMTP OUT :
RSET

17/09-2014 09:59:12 SMTP IN :
Disconnected

Has anyone an idea how i can fix the error.
It Hangs on an email they i can´t delete.