If VPOP3 has a temporary problem sending a message it will retry it later. When it will retry it depends on how many times it has already tried to send it, and is based on the 'Sender Retry' setting in the Mail Connectors → Mail Senders → Advanced tab.

If you want to force VPOP3 to retry sending messages immediately you can either do it on an individual basis, or on a set of emails.

1. In the VPOP3 settings, go to Users, then click on the Outgoing Message Queue link at the top of the page.
2. Double-click on the message you want to retry and check the Retry box in the top section, then close the message view window.

VPOP3 will retry the message when it next connects to send messages.

1. In the VPOP3 settings, go to Users, then click on the Outgoing Message Queue link at the top of the page.
2. Click on the Bulk Action button at the top of the page.
3. In the Action to perform box, choose Force Retry.
4. Enter search criteria (or leave blank to select all messages).
5. Check that the Messages Affected number looks reasonable.
6. Press Process Action to mark the messages to be retried immediately.

VPOP3 will retry the message when it next connects to send messages.