Trends AOL Accidentally Rejects Millions Of Messages By Ken Simpson | 2 minute read AOL’s mail servers recently started issuing permanent failure notices with an encouragement for the sender to “try again later”: 521 5.2.1 Service unavailable. Please try again later. Normally, if a mail server wishes to temporarily reject a connection or message, it will respond with a 400-series error such as “421 Try again later”. When the sending mail server sees this error code, it’s supposed to queue the message up and try again later. When a 500-series error is encountered — regardless of the text of the error message — the sending server is supposed to stop trying to send the message, and generate a bounce message to the original sender. The impact of this change, whether AOL intended it or not, is that many users (potentially millions) have received non-delivery receipts with “521 5.2.1 Service unavailable. Please try again later.” in their inboxes, rather than merely experiencing a brief delay in message delivery to their friends at AOL. What can I do if I get this error? Not much, unfortunately, other than to try sending the message again. Hopefully AOL will transform these 500-series errors into 400-series errors, and the mail servers of the world can resume queueing on our behalf when AOL’s mail servers are overloaded.