One of our clients is a non-profit organization that's asking if we can provide a mail hosting solution for their 30,000 or so members.

This needs to have the following features -
  • SMTP/POP3/IMAP/Web mail access
  • Web mail branding
  • The ability to automate turning individual accounts on and off as members renew (or fail to)
  • Spam filtering / antivirus
  • Addresses in a subdomain of their domain ([email protected])
  • Password integration with main site.
  • Single login with main site.
The information on whether a member is active or not is stored in a SQL Server 2005 database, as is the stored procedure to manage logins (which set a cookie).

We're looking for advice as to mail packages that can provide the basics and have hooks where we could integrate the custom pieces (integration with their member database and login) that can scale to 30,000 users. Has anyone run into such a beast?