[Postfix](http://www.postfix.org) | Mail Transfer Agent (MTA)
[Dovecot](http://www.dovecot.org) | POP3, IMAP and Managesieve server
[Apache](http://httpd.apache.org), [Nginx](http://nginx.org) | Web server
[OpenLDAP](http://www.openldap.org), [ldapd(8)](http://www.openbsd.org/cgi-bin/man.cgi/OpenBSD-current/man8/ldapd.8?query=ldapd&arch=i386) | LDAP server, used for storing mail accounts (optional)
[MySQL](http://www.mysql.com), [MariaDB](https://mariadb.org), [PostgreSQL](http://www.postgresql.org) | SQL server used to store application data. Could be used to store mail accounts too.