Why is SMTP Error 452 Insufficient system storage ? >> Error during mail Transfer ( Insufficient system storage )  & How to fix ?

Invalid Target (non-existanace emails): please validate the mailing-lists before using. If you send a-million invalid emails, the result will return to SMTP Server, the HDD will be full

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Message undeliverable

How to fix: >>> Please contact admin. I will delete the sent emails from the server and now You will have more space