So, a private cloud.
What to include in said cloud?
Well, let's start with another cloud, [Sovereign][]:
- IMAP over SSL via Dovecot, complete with full text search provided by Solr.
- POP3 over SSL, also via Dovecot
- SMTP over SSL via Postfix, including a nice set of DNSBLs to discard spam before it ever hits your filters.