One work around for this, if your ISP permits it, is to forward, at the ISP server, every arriving message to a second account while leaving the original message in place.  Not all ISPs offer this facility, but I seem to recall that GMail does.  You could then extract the mail through the Mercury POP process from one ISP account, while leaving the other one untouched.  I would have to check further about this but, in the GMail case, it might be better to use IMAP for your incoming connection.  I can't remember what all of the GMail options are.

If your ISP doesn't provide this type of forwarding facility, it may be possible to forward all messages from Mercury to a second account at your ISP.

 GordonM

I'm looking to use mercury as a local backup for my personal mails on yahoo. As such it would be really nice if i could simply have it fetch all mails from there and store them in its own mailstore. However on test runs it seems that it not only retrieves, but also deletes the mails on the remote pop3 server, which completely defeats the purpose of it being a backup.

Is there a way to tell it to leave the messages there?

No