My demonstrator Small Organisation Server now has working email, set up, as described, in overview at https://wordpress.debian.social/jlines/2020/12/02/ambridge-garden-club-email/
Pretty much all the components came from software packaged in Debian, and I think, even though the current setup went down a number of blind alleys, it should only need, for the email setup, the domain name to be served (which is the normal system domain name) and users having standard fields in an LDAP schema. There is quite a lot of overlap with Freedombox, although for flexilbity of LDAP administration it currently has phpldapadmin and FusionDirectory installed. I also had GOSA installed, but at present it conflicts in too many places with FusionDirectory. John
