Troubleshooting 5.4.4 Email Errors
Email sent to a particular domain may fail with the following error:
However, the receiver's domain is valid, and the MX is configured and resolves correctly. Furthermore, almost all email to the receiver is delivered correctly.
This may occur if the receiver's MX record is a CNAME. This configuration often works, but is not supported.
In this situation the sender's mail server does not fully resolve the MX to the final IP destination.
To resolve, change the MX to mail.anotherhost.com.
Note the changes will not take effect until the DNS timeout has expired on the MX record or the entire zone (usually 1-4 hours, but possibly up to 24).
In particular, MailChimp may generate “soft bounces” for domains with such an MX record.