Message Not Delivered: Email system is full
The message could not be delivered because the recipient's email system is out of space and cannot store any more messages.
Wait a few minutes, double check the recipient's email address and try sending the message again.
If this error continues, consider contacting the recipient through an alternative email address or other means.
5.3.1: Official Definition
A permanent failure is one which is not likely to be resolved by resending the message in the current form. Some change to the message or the destination must be made for successful delivery.
Mail system storage has been exceeded. The general semantics imply that the individual recipient may not be able to delete material to make room for additional messages. This is useful only as a persistent transient error.