Courier is a mail transfer agent (MTA) and groupware server. It implements IMAP, ESMTP, POP3, LDAP, SSL, HTTP, iCal. It also provides mailing list management, filtering, maildirs storage, and a webmail interface with calendaring and scheduling.
Postfix Admin is a PHP/web interface for managing mailboxes, virtual domains and aliases, fetchmail, and away or auto-reply messages. It requires MySQL/MariaDB or PostgreSQL, and optionally integrates with Squirrelmail / Roundcube.
SSHGuard is an automated log watcher which quickly sets up firewall blocks for detected brute-force attacks. It supports not just SSH, but also sendmail, exim, dovecot, Cucipop, UWimap, vsftpd, proftpd, pure-ftpd and FreeBSD ftpd. It understands syslog/-ng, metalog, multilog and raw log formats. And works with netfilter/iptables, PF, ipfw, or just hosts.allow to set up firewalling rules.