Have a look at this thread and see if one of the relaying options might work in your case:

http://community.pmail.com/forums/thread/22614.aspx 

Hi guys and gals,

I know this may not be Mercury specific, but I have had a couple of catchall accounts DM=localuser internet.domain.com style.
Is there a tool or a way to I guess resend the messages in the catch all account?

I want to remove the catchall and setup individual accounts now, and wish to "re deliver" the messages..

BUT the catch.....

I have 4 catchall accounts, 3 of them will remain on my current Mercury server, 1 of them (and this is the one I want to resend) is a new mail server on a different host.

Does anyone know if this is possible?

Cheers
Adam