SMTP Error Code: 421 4.7.2

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Lookup for "421 4.7.2"

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 (4.7.2)

Mailing list expansion prohibited

The sender is not authorized to send a message to the intended mailing list. This is useful only as a permanent error.