[Exim] frozen unroutable address'?

Hello..

Ugraded to Exim 4, and I'm seeing a few of these in mainlog:

2004-03-15 17:29:25 1B2ZLd-0005ol-Ld Unfrozen by forced delivery
2004-03-15 17:29:25 1B2ZLd-0005ol-Ld ** c.sears_uc@???:
Unrouteable address
2004-03-15 17:29:25 1B2ZLd-0005ol-Ld Frozen (delivery error message)

What have I done wrong so that unroutable address' are sitting here stuffing
up my queue?

Thanks,

Tim

P.S. I can send bits of my config, but not wanting to spam the list ...