My stack is postfix, dovecot, slapd for accounts, SoGO for web mail, calendar and task and contact management. Syncs to my phone via davx and just works out of the box. It's multi domain and my small company even sells hosted email services.
Rspamd for anti spam and dkim. Use a free email testing service to confirm SPF etc are setup correctly.
Also make sure you have regular backups and up to date lets encrypt certificates.
Strange coincidence right as they want to convert to a for-profit company structure. "Bro we are not even making profits, nothing to see here bro"