Symptoms
Many messages like these are sent to Plesk administrator's email:
Out: 220 example.com ESMTP Postfix
In: MAIL From:server@example.com
Out: 452 4.3.1 Insufficient system storage
In: RCPT To:admin@example.com
Out: 503 5.5.1 Error: need MAIL command
Cause
Plesk server is out of disk space:
# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/ploop12525p1 9.8G 9.8G 0G 100% /
none 512M 4.0K 512M 1% /dev
none 512M 0 512M 0% /dev/shm
Resolution
Free up some disk space (for example, clean up temporary files: How to clean temporary Plesk files on a Linux server) or extend the partition.
Note: managing disk partitions is an administrative task - please contact your hosting provider for further assistance.