Strange problem?

My queue started filling up mail well past the expire time.  What looks like is
happening is that a spammer tried to send to or relay through me and then
killed the domain. I suppose the domain could have been forged, but I do
reverses.  I had bounces set up for about 6 hours figuring that most legitimate
bounces would be delivered in that time.

I ran exim -qff -v and watched.  I was getting some very long timeouts.  I set
bounces to die after 0h and I am down from around 3700 to around 2500 after
about 8 hours. That includes me killing some of the older stuff manually.

I originally though I had caused the problem by changing the timeouts when I
first noticed the queue seemed to be growing.  Since this machine, at least
right now, is primarily a gateway, if something doesn't deliver right away,
then something else is wrong.

This sound plausible?

Hermit