Would turning off Cached Exchange Mode help with all these mailboxes?

First I’d check how the mailboxes are actually added. A mailbox opened through delegated access or automapping can be cached in the user’s OST, while separately added Exchange accounts may have their own OST files. The label “regular mailbox” doesn’t tell you which setup Outlook is using.

If they’re delegated mailboxes, try turning off the option to download shared folders, where available, and check whether a few mailboxes account for most of the growth. That may address the bloat without putting every mailbox into online mode.

Online mode does avoid building a large local cache, but the trade is that Outlook depends on the connection and Exchange being responsive whenever the user works. A brief service or network problem can affect access to all the mailboxes, and opening or searching large folders may feel slower.

I’d test it with one user first, especially during the parts of their workday when they handle the busiest mailboxes. If it feels laggy or unreliable, switching back is straightforward, but it’s worth checking the user’s workflow before making it the default fix.

I wouldn’t assume that one giant Outlook profile is the only workable setup. If they don’t need every mailbox open all day, using web access for the less-used ones can keep them out of the local cache. A separate profile or separately added account may also be worth testing, though it can mean more switching and its own setup overhead.

The newer Outlook app is another possible trial if the user’s required features are supported there. I’d treat that as a compatibility check, not an automatic upgrade, since some workflows still depend on options that differ from classic Outlook.