SMTP Error Code 551

The transaction failed permanently. The server will not try to send the message again.

Providers

ImprovMX · Yandex

ImprovMX↑

551 5.7.1 Due to the very low reputation of the sending domain, this message has been blocked (Code 2).

Your domain is on a reputation blacklist such as the Spamhaus DBL. You will need to be delisted before you can send emails.

551 5.7.1 Loop detected: Email is sent and forwarded from/to improvmx.com. (#id-5.9.0)

The destination mailbox loops back to ImprovMX. They block these messages in order to avoid loops.

551 5.7.1 Domain not found (#id-5.9.2)

The domain was not found in the ImprovMX database.

551 5.7.1 Our database returned invalid pieces of information about the requested recipient’s domain (#id-5.9.3)

551 5.7.1 Unable to correctly read the forwarding recipient (where to forward the email to) (#id-5.9.5)

Yandex↑

551 5.3.0 Unknown error