We have a single SMTP server that handles all the incoming/outgoing e-mail for our school district. It's the primary MX for the district. It runs postfix, spamassassin, amavisd-new, clamav, mysql.
It handles between 2-5 million messages per month, for approx. 15,000 e-mail addresses (students and staff), and about 90% of those messages are blocked as spam (so they never leave this box).
However, this box only handles SMTP traffic. There are no user accounts on this system, there are no mailboxes on this system. It forwards messages on to either the main staff Zimbra server, or to the school webmail servers, or to other servers as needed. This is just a relay/scanning box.
All outgoing e-mail from all servers and clients in the district is funnelled through this box, either via direct settings in the clients, or firewall redirects.
This is an older AMD Opteron 200-series system with 2x single-core CPUs and 6 GB of RAM. More modern hardware would be able to handle even more SMTP traffic.