Email provider
ImprovMX SMTP Error Codes and Bounce Messages
ImprovMX is an email forwarding service that receives mail on a custom domain and passes it to a personal inbox. Its rejections are documented in a public error-code guide.
- Codes on record
- 3
- Responses
- 7
Postmaster & documentation
SMTP 4501 response
A temporary problem. The sending server keeps retrying for a while, and delivery often recovers on its own once the underlying issue clears. See SMTP 450 across every provider
SMTP 5501 response
A hard failure. The receiving server has refused the message for good and will not accept an unchanged retry without a change on the sending side. See SMTP 550 across every provider
SMTP 5515 responses
smtp;551 5.7.1 Due to the very low reputation of the sending domain, this message has been blocked (Code 2).
smtp;551 5.7.1 Loop detected: Email is sent and forwarded from/to improvmx.com. (#id-5.9.0)
smtp;551 5.7.1 Domain not found (#id-5.9.2)
smtp;551 5.7.1 Our database returned invalid pieces of information about the requested recipient’s domain (#id-5.9.3)
A hard failure. The receiving server has refused the message for good and will not accept an unchanged retry without a change on the sending side. See SMTP 551 across every provider
ImprovMX bounce questions
›Why is ImprovMX bouncing my email?
ImprovMX is an email forwarding service that receives mail on a custom domain and passes it to a personal inbox. Its rejections are documented in a public error-code guide. Match the exact wording of your bounce to one of the 7 responses on this page to find the specific reason and fix.
›How do I improve deliverability to ImprovMX?
Authenticate with SPF, DKIM and DMARC, keep your sending IP and domain reputation clean, mail only engaged recipients, and honour unsubscribes. ImprovMX also publishes postmaster guidance, linked on this page.
