SMTP Error Code: 421 5.7.9

what's your mail server saying?

Lookup for "421 5.7.9"

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 (5.7.9)

Authentication mechanism is too weak

The selected authentication mechanism is weaker than the server policy permits. Client should retry with a new authentication mechanism.