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!
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