Make courier only deliver one message at a time

I'm currently running courier-mta (along with courier-imap and courier-pop3 and such), with mail delivery happening through maildrop, which uses spamassassin via spamc.

This works all well and good, but one of my user's accounts gets a lot of spam. A LOT. And when spamassassin tries to process more than one message at a time, things slow to a crawl on my Linode 80. (Other than that, though, performance is fine.)

How do I make it so that maildrop or courier won't try to deliver more than one message at a time? I can't just cut down on the number of spamassassin daemons running, because if maildrop can't get an answer from spamc within a specified time, it just bypasses the spam checking. I just need to keep courier from trying to deliver more than one message at a time.