dovecot: dovecot (IMAP / POP3 server written with security primarily in mind)
dovecot:
dovecot: Dovecot is an excellent choice for both small and large installations.
dovecot: It's fast, simple to set up, requires no special administration
dovecot: and it uses very little memory. It's among the highest performing IMAP
dovecot: servers while still supporting the standard mbox and Maildir formats
dovecot: It works with NFS and clustered filesystems and is admin-friendly.
dovecot: User authentication is extremely flexible and feature rich.
dovecot: It supports workarounds  for several bugs in IMAP and POP3 clients
dovecot: Postfix 2.3+ and Exim 4.64+ users can do SMTP authentication directly
dovecot: against Dovecot's authentication backend