This error is generated by the destination mail system, which might be out of disk space. According to Request for Comments (RFC) 821, Appendix E, this error is a mail-specific error and message delivery should be retried later.
The following is an excerpt of relevant text from RFC 821:
4yz Transient Negative Completion reply
The command was not accepted and the requested action did not occur. However, the error condition is temporary and the action may be requested again. The sender should return to the beginning of the command sequence (if any). It is difficult to assign a meaning to "transient" when two different sites (receiver- and sender- SMTPs) must agree on the interpretation. Each reply in this category might have a different time value, but the sender-SMTP is encouraged to try again. A rule of thumb to determine if a reply fits into the 4yz or the 5yz category (see below) is that replies are 4yz if they can be repeated without any change in command form or in properties of the sender or receiver. (E.g., the command is repeated identically and the receiver does not put up a new implementation.)
x5z Mail system
These replies indicate the status of the receiver mail system vis-a-vis the requested transfer or other mail system action.