Delayed Message: Email system is full
The email message has not yet been delivered because of an issue with the recipient's email provider.
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.
4.3.1: Official Definition
A persistent transient failure is one in which the message as sent is valid, but persistence of some temporary condition has caused abandonment or delay of attempts to send the message. If this code accompanies a delivery failure report, sending in the future may be successful.
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.