Hello Fauzi,
The queue structure in SMTP5 is the same as any IIS structure. And largely the mail can be manipulated in the same way. If you to go the "mta" folder in our directory structure you will see our queues. When you look through these folders there will be files with blank extensions. (These are your email files.) You can relocate these files and effectively clear the queue.
Please make sure that you do not delete any information without knowing what it is that you are getting rid of. Also, do not delete the folders as this can cause issues with the queue structure itself.
Thanks!