SMTP Error Code: 421 4.3.5

what's your mail server saying?

Lookup for "421 4.3.5"

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 (4.3.5)

System incorrectly configured

The system is not configured in a manner that will permit it to accept this message.