All mail stopped flowing on a Microsoft SBS 2003 box with Exchange 2003 running the POP3 downloader.  Small Business Server 2003 Exchange routing services spat errors:

Event Type:      Error
Event Source:   MSExchangeSA
Event Category:            RFR Interface
Event ID:          9074
Date:                21/02/2005
Time:               8:43:18 AM
User:                N/A

Description:
The Directory Service Referral interface failed to service a client request. RFRI is returning the error code:[0x3f0].

The mail is POP3ed from an ISP.

Cause: There were two NICs on the box.  The second disconnected NIC didn’t have File and Printer Sharing enabled, as per http://support.microsoft.com/default.aspx?scid=kb;EN-US;279742

You think they would have come up with a workaround by now.  Such a dirty solution.  The NIC didn't even have an IP bound to it.