If the value of 'Mailbox size' on a subscription is set as a certain value according to Service Plan (e.g. 50Mb) the mail messages sent to mailboxes reached the 50Mb limit are received successfully.
The cause is 'Overuse policy' setting on a subscription.
If 'Overuse Policy' is set as 'Overuse is allowed (not recommended)' the mail messages will be received independently of the 'Mailbox size'. It is required to set 'Overuse Policy' as 'Overuse is not allowed' or 'Overuse of disk space and traffic is allowed', in this case all mail messages sent to mailbox reached the limit will be rejected.