SMTP Error Code: 421 5.7.6

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Lookup for "421 5.7.6"

Permanent Error - This status code indicates a permanent issue and is unlikely to resolve itself. You should contact the Email Service Provider.

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 (5.7.6)

Cryptographic algorithm not supported

A transport system otherwise authorized to validate or decrypt a message was unable to do so because the necessary algorithm was not supported.