What does a 4xx or 5xx error message indicate?
This article describes how to handle 400 or 500 series error messages when sending emails.
An SMTP error, or a Delivery Status Notification (DSN), indicates that an email could not be delivered, either due to a temporary or permanent problem.
A 4xx error indicates a temporary issue. It can occur if, for example, 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).
A 5xx error indicates a permanent failure. You should not retry sending an email that comes back with such an error.
While a 5xx error may be generated for something as simple as trying to send a message to an invalid email address, it may also be indicative of a more widespread, general problem.
We encourage you to investigate further if you consistently receive 4xx or 5xx errors when sending to Yahoo!. Our best practice recommendations, is a great place to start. List managers should have a policy for removing email addresses that generate errors/bounces.