Vis a vis the email part, I would think it possible that it should be pretty easy to replicate IMAP locally.

I haven't used this in several years, but have installed and used a system called offlineimap (see http://www.offlineimap.org/) which made it relatively easy to duplicate IMAP repositories on a per user basis.

It needed only a couple lines of configuration per source.  And note it is available as a Debian package :-)
https://packages.debian.org/search?keywords=offlineimap

There are sophisticated IMAP servers that are complex to manage, but this was pretty easy.  If it covers your needs, then it's a pretty good answer.

I'd like to try out one of the options that uses Postgres as back end; there should be big storage savings available via email contents being automatically compressed in "TOAST" columns, but that is certainly a way more complex approach.  Offlineimap is pretty simple!