For self-host: - iredmail if you want it simple - dovecot + postfix + opendkim + opendmarc + bogofilter + rainloop if you like tinkering and you have a small

For self-host: - iredmail if you want it simple - dovecot + postfix + opendkim + opendmarc + bogofilter + rainloop if you like tinkering and you have a small/personal setup. Drop in radicale with imap auth for calendar/contacts, and prosody with imap auth for jabber, and you're good for life, unless people want shiny web interfaces, in which case consider nextcloud for contacts/calendar. - zimbra as an all-in-one solution - exim + cyrus if you want mail only for a large company and you want it open source