4xx or 5xx SMTP error message notifications
An SMTP error, or Delivery Status Notification (DSN), indicates that an email could not be delivered, either due to a temporary or permanent problem. You can review our complete list of SMTP error codes for specific details about each error.
A 4xx error indicates a temporary issue. It can occur for multiple reasons including, the Yahoo MTAs are busy when a connection is attempted, or if there is a dynamic deprioritization in effect for a sender's server. If you see this error when trying to send an email to Yahoo Mail, you may retry sending at a later time (retrying a few times if the 4xx error persists). Learn more about 4xx SMTP errors and why you may have received one.
A 5xx error indicates a permanent failure. You should not retry sending an email that comes back with such an error.
5xx errors can be generated for something as simple as trying to send a message to an invalid email address. Usually they're a symptom of a more widespread, general problem. Learn more about what each 5xx SMTP error means and the best way to prevent them in the future.
We encourage you to review our best practice recommendations if you consistently receive 4xx or 5xx errors when sending to Yahoo. List managers should have a policy for removing email addresses that generate errors/bounces.