SMTP Error Code: 421 4.2.0

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Lookup for "421 4.2.0"

Temporary Error - This status code indicates a transient issue and may resolve itself.

Basic Status Code Details (421)

Service not available, closing transmission channel [This may be a reply to any command if the service knows it must shut down]

Enhanced Status Code Details (4.2.0)

Other or undefined mailbox status

The mailbox exists, but something about the destination mailbox has caused the sending of this DSN.