[exim] Whitelisting addresses/hosts my server has sent to

At $DAYJOB I operate a small corporate mail server-- roughly 70 users,
roughly 1000 legitimate incoming messages per day and 3-5 times that many
rejected as spam via Exim ACLs and SpamAssassin. Messages scoring 10+ on SA
are rejected at SMTP time, messages scoring between 5 and 10 points (10-50
per day) are dropped in a spambucket for me to review. All is working well.

Because I am lazy, and because we work in a time-sensitive industry, I want
to spend less time reviewing quarantined email. One way I think I can reduce
the false positives (mainly somebody's friends or family forwarding joke
emails with lots of pictures or lots of forwarded URLs) is by assuming that
users and/or hosts that my users have emailed are less likely to be spammy.
Specifically, I plan to have Exim track a list of recipient addresses and
hosts and make sure that the hosts bypass blacklist checks and the users
have an X-Known-Sender: header added that SpamAssassin will then recognize
as a hamsign.

(I recognize that for many ISPs the MXes will have no bearing on the sending
hosts. That's okay; I'm not blacklisting other hosts, they just don't get
the "get out of blacklists free" card that hosts that have received our mail
get.)

I ran through the logs for January through April, and found 17643 deliveries
to 4383 unique addresses and 1844 unique hosts. (These numbers are wildly
skewed by the fact that most outgoing mail was not going through Exim until
the third week of March.) I have not yet done any analysis of how many new
unique recipients are added each month, but it seems unlikely that would be
more than 10%.

Given those ballpark numbers, I assume that I'd find some sort of SQL
database my best bet for tracking these "known sender" and "known host"
databases. Is this correct, or is there a rule of thumb to tell me when an
lsearch would be more efficient?

Has anybody else put a system like this into production? Any experiences or
pitfalls to share?
--
Dave Pooser
Cat-Herder-in-Chief, Pooserville.com
"...Life is not a journey to the grave with the intention of arriving
safely in one pretty and well-preserved piece, but to slide across the
finish line broadside, thoroughly used up, worn out, leaking oil, and
shouting GERONIMO!!!" -- Bill McKenna