[exim] Mail forwarded from other hosts missing!

I currently host the mail of several virtual domains using exim4.

When mail is sent from shawn@??? to shawn@???, it
comes through right away without delay.

The problem into which I am running is when mail is sent from
shawn@??? to shawn@??? -- an account which is
.forwarded to shawn@???.

These messages never arrive. Strangely, they never bounce back, either.
I've checked /var/spool/exim4/msglog/ and /var/log/exim4/ and nothing
unusual appears there. The FAQ's don't seem to address this, the list
archives don't seem to turn up anything, and general web searches have been
fruitless. I assume it has something to do with exim's routers, but I'm not
sure what needs to be done.

Does anyone have any suggestions?

Thanks!
Shawn