I don't think there is a "direct" way to do this, as the POP3 client is instructing the SMTP server to remove email, which it dutifully does.

 

Workarounds would include using filters to copy the incoming email to another mailbox under that user or copy the incoming email to another user entirely. Another workaround is to have the POP3 client actually NOT delete emails from the server, and find a way to delete emails from that client on some kind of schedule.

 

If you have archiving available in SM, you could always retrieve email from the archive, but that's pretty much "on-demand" and might not be what you're looking for.