I would like your precious feedback on the "Email archiving" subject. The need is to keep all domain emails (Inbox, Sent + User folders) archived centrally for future reference.

Current setup

Domain email is setup on @jarland 's awesome service. Six users (currently but expecting the number to grow) connect via Microsoft Office Outlook POP3 and download their emails. All messages are saved locally on Outlook .PST (which is a PITA to manage and backup). This setup is problematic both because of PST management and issues with mobile devices (IMAP devices don't see Sent emails that were sent from Microsoft Office Outlook).

What I would like to accomplish

1. Keep Microsoft Office Outlook (since they don't want to switch to another email client) and setup all clients to use IMAP.
2. Setup a "solution" that will archive all emails for all users on a device within the business (not on a cloud service). Older emails (definition of older to be decided) will be removed from the IMAP server and only kept on archive.
3. Users or/and the IT admin will need to have access to the archive, whenever needed.
4. The "solution" should be able to archive directly from IMAP server (all main folders like Inbox, Sent, ect + any user created folders)
5. The "solution" should be able to be scheduled. Probably quite a few times per hour.
6. I would prefer a "solution" that does not require to create a catch-all mailbox. Just the users' mailboxes (no need to worry about changing passwords, because the users can not change it).
7. For starters the "solution" should support 10 users (expecting to grow)
8. Easy to backup the "solution" (like copy files, (my)SQL backup, etc. NOT something ugly like .PST)
9. The "solution" should be able to work without any email client installed on the machine (since it will not be installed on users' computers).
10. Linux or Windows (but preferably Windows)

I am evaluating both free and paid solutions.

What I have seen so far:

https://www.mailstore.com/en/products/mailstore-home/
Pros: Free, Windows, Really (really!) easy to use, Easy backup of archived data, additional feratures
Cons: Scheduling via Task Manager is impossible (or very hard even - found the way to run it through TS but the process remains running (so it doesn't run the next schedule).

https://www.mailstore.com/en/products/mailstore-server/
Pros: Same as above
Cons: 295€ for just 5 users and the price increases as the number of users grow

https://www.mailpiler.org/
Pros: Professional email archiving, Open Source
Cons: Needs extensive setup, needs addition catch-all mailbox, not very user friendly, needs resolvable hostname on the running mailpiler server

imapsync
Pros: Easy to setup
Cons: Needs to setup an additional IMAP server (within the business) to copy messages to, which I would rather avoid.

How would YOU handle such a case? Thoughts, opinions, discussion....