Well, the problem is those error messages are not indicating there is extension 5615 or 4765 - those are message IDs.
The first message indicatess the mailbox attached to the user with extension 4003 is full. That's it. The 5615 is a message ID, not an extension. All this error is indicating is that user sent a message and since his mailbox is full, Exchange will not allow us to send it on their behalf so we have to send it as an unknown caller to get around it. This is a warning to let you know that you need to get that guy's quota up or have them clean up their inbox. We cannot get around the Exchange inbox limit enforcement rules.
On the 2nd error, the only extension noted in there is [1194] - the 4765 is just a message Id number - it's not an extesnsion. As the error message states, the user called from 1194 and entered an invalid PW when attempting to get into that mailbox (this is for tracking crackers who may be hitting your system).
