This code may be caused by a resource problem such as a disk full condition. This code may also occur if your Simple Mail ...

{4}: This code may be caused by a resource problem such as a disk full condition. This code may also occur if your Simple Mail Transfer Protocol (SMTP) queue is on a File Allocation Table (FAT) partition and the service has reached a Windows-imposed limit on the number of concurrent file handles that can be opened by the SMTP Service. In this case, instead of receiving a disk full error, you might be receiving an out of memory error.