mail server - smtp/pop3/imap, antispam (popfile bayes xmlrpc server, learning through imap/http; spf, senderid, rbl, yahoodomainkeys), antivirus (integration with 5 server antivirus api, including clamav), content filter, aliases, routing, mail lists, mail robots, control web-interface, webmail, statistics, unlimited domains and users number, any methods of authorization (activedirectory, winnt local, odbc, text lists..). opensource, plugins api.