A patch for MercuryP v4.51 is now available at http://community.pmail.com/files/folders/patches/entry2517.aspx.
The patch addresses the UIDL response to issue a NUL list instead of issuing an error when a maildrop is empty.
The RFC regarding this response is vague, however the previous error response from Mercury when the maildrop is empty causes some PDA POP3 implementations to fail.
Kind regards / Peter