[Exim] exim fine-tuning

Hi,

last weekend I experienced a DoS *cheer*. In fact this wasn't as funny
as I expected, since I ended
up with a pretty busy mailserver, who rejected a lot of messages with
"Connection from x.x.x.x refused: too many connections" (where x.x.x.x
stands for funny ips/hostnames
from .cz or .pl). The box was pretty under load (every mail forked a
spamc-process aso...). I rebooted the
box and changed the smtp_accept_max from 20 to 120 and today I changed
local_scan_timeout to 45s.
Since I don't have pretty much experience with exim as productive
system, I'd like to know if doing those
changes is "a good way" to prevent me from driving to work at weekends.
Are there any other fine-tunings
I should do to my mailserver to make it even more flexible, funky and
invulnerable?

thx

-fe

P.S.:
exim 4.21
sa-exim 4.0
sa 2.63