[Exim] Allowed rcpthosts?

Hiya. I've had a .forward set up for one user for some time, now, and
until this weekend, all worked fine. Nothing's changed that I can tell
(though I'm checking with the owner of the remote site to be sure). The
mail bounces with "553 sorry, that domain isn't in my list of allowed
rcpthosts (#5.7.1)". I've tried to figure out where the "rcpthosts"
would be defined, and have, thus far, failed. Any pointers on how to
make sure that this isn't an issue on my side?

Thanks, and sorry if this is a fairly obvious question...

Ken D'Ambrosio
Sr. SysAmdin,
Xanoptix, Inc.