When using a catch-all or multi-user POP3 mailbox, a single download may contain emails for many different recipients. POPcon must decide which recipients are local (to be delivered to Exchange) and which are not.
Without a list of accepted domains, POPcon cannot filter correctly and may attempt to deliver to unknown addresses, causing Exchange errors or re-routing everything to the postmaster.
Enter each domain you want POPcon to handle (e.g. yourcompany.com) in the Accepted Recipient Domains field. This tells POPcon: "only deliver emails where the recipient address ends in one of these domains."