[Exim] rejected relay

I'm looking for some clarification on the following log entry:

1999-07-27 08:20:35 refused relay (host accept) to <netgroup@???> from <711127@???> H=(btin) [216.13.117.120]

What confuses me is the 'refused relay (host accept)' part. How does
refused relay and host accept work together?

Thanks,

----------------------------
Dave Dunaway [bela@???]