Can Exchange delete mail after 90 days but keep messages in certain folders?

Exchange messaging records management is one option to investigate. You can create retention tags for the default folders, add them to an Exchange retention policy, and assign that policy to the relevant mailboxes.

The tricky part is the Keep subfolder. Don't assume it will be exempt just because users created it. Check how tags are inherited and whether an appropriate personal tag can be applied to that folder. Test the exact folder structure before rolling anything out.

Can the keep folders live at the mailbox root instead of under Inbox? That might make the setup easier to reason about. You would still need to verify the tags and inheritance, but it gives users a clearer separation between short-lived mail and mail they want to retain.

Purview retention policies and labels are another possible route, but they aren't interchangeable with Exchange folder tags. A mailbox-wide deletion policy plus a label that retains selected items could protect those items for the label's retention period. Check the interaction rules and whether the label can be applied the way your users need before treating it as a folder-based exception.

Also clarify what preserved forever means. Keeping mail for a longer period is not necessarily the same as configuring indefinite retention.

Watch the age of messages when testing. If someone moves an already old message into a folder with a 90-day deletion tag, it may be eligible for deletion when the Managed Folder Assistant processes it. The move doesn't necessarily give it another 90 days.

Use a test mailbox and check both newer and older messages, including moves into and out of the Keep folder. Make sure compliance understands the recovery options before enabling deletion broadly.