To delete messages from the VPOP3 'OutQueue':

1. log into the VPOP3 settings
2. go to the Users page
3. click on the OutQueue entry in the accounts list. This will show you a list of all the messages waiting to be sent by VPOP3
4. Select the messages you want to delete, and either Hold or Delete them. Holding a message will leave it in the OutQueue, but VPOP3 will not try to send it. Deleting a message will remove it from the OutQueue permanently.

Note that you cannot delete items from the VPOP3 OutQueue whilst VPOP3 is in the process of sending them, so you should stop VPOP3 from sending first.

If there are too many messages in the outqueue and you cannot delete them using the above method, you can delete all the messages them the 'backdoor way'. To do this, stop VPOP3, then delete all the Q*.DAT files in the VPOP3\Outqueue directory, then restart VPOP3. (Note that doing it this way, you can't be selective about which messages you delete).

If you want to save some messages, you could move the Q*.DAT files into another folder on the disk, then move them back in smaller chunks, deleting the unwanted messages as you go.