I was expecting a more verbose anwser :-). The "shupp" toaster uses qmail-pop3d but I have seem others that use courier-pop3d ("lazydog" toaster - www.qmail.nu if I remember right). Is there a reason for that? performance maybe ?
I think qmail-pop3d is secure. Anyway, I don't use quotas and use dovecot as my imapserver because it is faster and works better than courier-imapd.
-- Eero