Hi TpicTech,
If by "email trails" you mean email archival (and not login history and such), then:
LuxSci Premium Email Archival will keep it for 10 years (which is more than any legal requirement). Email is not available via folders, but via an archival portal.
They can get an account and, via WebMail, share some or all folders with any or all users in their account and then also not delete the email. This will have the effect of keeping the email as long as they need and sharing when they need with whom.
Caveats: Shared folders are shared in webmail only -- not IMAP or MobileSync. Messages kept in user accounts (as opposed to Archives) are deletable -- so maybe they want to get both. Disk space will be a factor after a while. So they would probably want a dedicated solution if the storage is expected to grow beyond 100s of GB.
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