Email provider · 5.x.x permanent failure
Every SMTP 551 response on record from ImprovMX, with the verbatim text, the enhanced status code, a severity rating and a plain-English fix.
The recipient is not local to this server and no forwarding path is offered. It is permanent, so send to the address the recipient actually uses.
smtp;551 5.7.1 Due to the very low reputation of the sending domain, this message has been blocked (Code 2).
The receiver's spam filter scored the message above its rejection threshold. Review the content and links, build sending reputation gradually, and only mail people who asked to hear from you.
smtp;551 5.7.1 Loop detected: Email is sent and forwarded from/to improvmx.com. (#id-5.9.0)
The receiver refused the message for good and points at a policy or security problem (enhanced status 5.7.1-2). Use the wording above to pin down the exact fix.
smtp;551 5.7.1 Domain not found (#id-5.9.2)
The receiver refused the message for good and points at a policy or security problem (enhanced status 5.7.1-3). Use the wording above to pin down the exact fix.
smtp;551 5.7.1 Our database returned invalid pieces of information about the requested recipient’s domain (#id-5.9.3)
The receiver refused the message for good and points at a policy or security problem (enhanced status 5.7.1-4). Use the wording above to pin down the exact fix.
smtp;551 5.7.1 Unable to correctly read the forwarding recipient (where to forward the email to) (#id-5.9.5)
The receiver refused the message for good and points at a policy or security problem (enhanced status 5.7.1-5). Use the wording above to pin down the exact fix.
Postmaster & documentation
The recipient is not local to this server and no forwarding path is offered. It is permanent, so send to the address the recipient actually uses. At ImprovMX, it is a permanent rejection, and the specific reason is in the enhanced status code and message text of each response below.
Match your bounce text to one of the responses below and follow its guidance. Recipient-side wording means fix the address. Authentication or reputation wording (SPF, DKIM, DMARC, blocklist) means fix your sending setup, and check ImprovMX's postmaster documentation.
