If you have the VPOP3 Mail Collector setting Leave Mail On Server enabled, then VPOP3 will leave your messages on the ISP POP3 server for the specified number of days after downloading it.
If, during this time, you want to make VPOP3 download those messages again, then you can do so.
If you have VPOP3 version 5, go to Settings → Mail Collector → (collector) → Messages tab. You can select the message(s) you want to re-download and check the 'Get' box for them. (Or check the 'Get' box in the table header to get all the messages).
If you have VPOP3 version 4 or earlier, you do this by following the procedure below.
Note that you cannot selectively download messages again using VPOP3 (there are other utilities available on the Internet to do this) - you have to re-download all the left messages again.
To find the numeric ID of the Mail Collector you want to re-download mail for, go to Mail Connectors in the VPOP3 settings, and select the appropriate Mail Collector. Look in the address bar of your web browser, and you will see something like:
http:/ /<server IP address>:5108/admin/index.html#connectors_206
Subtract 200 from the number after 'connectors_', and that will give you the numeric ID - in this example, that is '6'.
If you just wish to download a few messages again, you can avoid disruption to most of your users by using the following technique.