With the current version of SI, it is not possible to define a limit for users on mailboxes that disallows users to upload files once that limit is exceeded.
In some cases, our customers want to avoid users abuse their SI system. If a user can no longer upload a file due to this size limitation, he/she should delete an existing file in the mailbox and try again.
It should also be kept in mind that there are some complications such as submailboxes and more than one user having access to the same mailbox. So this limitation should be defined for a user on a specific mailbox (or a submailbox).