We use Debian for our mail hubs. Postfix/Clamav/spamassassin/SQLGrey filter all mails for all our ten research centers. We have two servers running for these tasks, and we chose Debian for its safety and stability. In a few weeks we'll migrate all this under Debian Squeeze on two virtual machines. Debian is also used for our main DNS servers (again, two servers for all research centers), and for the outgoing SMTP service.