SMTP Error Code: 421 4.5.6

what's your mail server saying?

Lookup for "421 4.5.6"

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 (4.5.6)

Authentication Exchange line is too long

Authentication failed due to the client sending a [BASE64] response that is longer than the maximum buffer size available for the currently selected SASL mechanism.