SMTP Error Code: 421 4.2.4

what's your mail server saying?

Lookup for "421 4.2.4"

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 (4.2.4)

Mailing list expansion problem

The mailbox is a mailing list address and the mailing list was unable to be expanded. This code may represent a permanent failure or a persistent transient failure.