Johannes Erdfelt of S.u.S.E wrote a patch to allow qmail to use a concurrency greater than 240.
If your queue gets larger than about 23,000 separate messages you'll need to change conf-split and recompile. Different Unix filesystems will affect that 23,000 figure, but not by more than a factor of two. This will ruin your queue, however. The best way to make this change is to run two copies of qmail like this:
rename /var/qmail to /var/qmail2.
change conf-qmail to /var/qmail2.
change conf-split to a prime number which reflects your requirements.
change conf-qmail back to /var/qmail
run /var/qmail2/rc. In a week you'll be deleting this copy, so don't worry about setting it up under daemontools.