.. I'd be wary of setting the maxspace too small; when you get an archive file that has to be unpacked for scanning it uses this value to decide how much disk space to use for the unpacking process, this doesn't affect the mail send/receive cycle directly.
You may wish to consider limiting the number of sessions that the server runs concurrently to allow more bandwidth per message during reception/sending (tho with 2Mb it shouldn't really be necessary):
running the following:- /sbin/e-smith/config show qmail
gives somthing like:
qmail=service
ConcurrencyLocal=10
ConcurrencyRemote=20
status=enabled
to change the concurrency values do (for example):
/sbin/e-smith/db configuration setprop qmail ConcurrencyLocal 5
/sbin/e-smith/db configuration setprop qmail ConcurrencyRemote 10
/sbin/e-smith/signal-event email-update
/etc/init.d/qmail restart
(Shamelessly stolen form a post by Ray Mitchell

)