Unless you limit the number of connections to each recipient host, you risk taking an email server hosting a large number of monks to its knees.

Qmail is a very good internal system. I have stopped recommending it for external mail servers a long time ago. The internet infrastructure is just not resilient enough against a server like this that could present a very effective denial of service attack.

This is from someone who was involved with qmail use and advocacy very early on. One of my first public perl scripts is in fact still being distributed (search for Brian T. Wightman on one of the qmail pages), although I no longer recommend its use - the SMTP world has changed :).