Shameless un-compensated plug:
I am now using Securence for security, continuity, archiving. This includes a web interface with Shadow mailbox that each user can access, to see/search their sent/received emails, not to mention viewing their Quarantine and being able to release/whitelist false positives. They can also blacklist senders if allowed (I don't allow it per user).
Also allows them to send/receive recent emails directly if their mail server is down. AV and Anti-Spam (etc.) are highly effective with VERY low false-positive rates. Other features too numerous to mention in this already too-long post.
Costs about a couple bucks/mo./user (as a reseller) but also allows email to flow to unregistered users, that you don't pay for (so catch-all still works, for example). I'm fortunate enough to have clients who don't mind paying for this stuff per-user.
In use now for several months. Extremely pleased with the service. Maintenance overhead close to zero once an account and users are set up.
Oh yeah, and 24x7 phone tech support. Whoohoo. A division of U.S. Internet, quite a large provider. Developed for their own use and then made available to others, including Fortune 100 companies.
Moved to Securence from Messagelabs/Symantec Cloud for more than one client and never looked back.
Environments are SmarterMail <--> Microsoft 365 and iMail Server <--> Microsoft 365 split-domain arrangements.
BTW the SmarterMail archive is great/useful but is not available directly to users and of course does not work with Microsoft 365 users. Microsoft's own archiving costs more, but my feeling also is that it's good to have another party handling this for continuity purposes.