On second thought, completely remove this check.

If the admin (unwisely) is not caching, that's their prerogative. That
shouldn't change where large items should be stored though (if cache
driver is null, they simply won't be cached).
[mms] Ensure persistently stored data is not overwritten by another session.

Key needs to be unique to the current session, or else a user with
multiple active sessions may overwrite the stored data.
[mms] Ensure that search/POP3 mailbox listings are saved even if there is an issue with the underlying cache backend.

See Ticket #13264
Released Horde_Core-2.13.1
Development mode for Horde_Core-2.13.2