SMTP Error Code: 421 4.1.3

what's your mail server saying?

Lookup for "421 4.1.3"

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 (4.1.3)

Bad destination mailbox address syntax

The destination address was syntactically invalid. This can apply to any field in the address. This code is only useful for permanent failures.