SMTP 551

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

ImprovMX blocked this for very low sending-domain reputation, often a DBL listing. Delist the domain, then retry.

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

ImprovMX blocked a mail loop back into itself. Fix the forwarding target so it does not point back at ImprovMX.

smtp;551 5.7.1 Domain not found (#id-5.9.2)

ImprovMX does not know this domain in its database. Confirm the alias domain is set up correctly.

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

ImprovMX has invalid forwarding data for this recipient domain. Fix the alias configuration in ImprovMX, then resend.

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

ImprovMX could not read the forwarding recipient. Fix the forward-to address in ImprovMX, then resend.

smtp;551 5.3.0 Unknown error

Yandex returned an unknown 551 error. Retry once. If it repeats, verify the address and contact Yandex support with the full bounce.