SMTP Error Code: 421 5.7.3

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Lookup for "421 5.7.3"

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 (5.7.3)

Security conversion required but not possible

A conversion from one secure messaging protocol to another was required for delivery and such conversion was not possible. This is useful only as a permanent error.