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An asynchronous bounce, also known as an async bounce, is a type of email bounce that occurs after the Simple Mail Transfer Protocol (SMTP) connection has been completed and the email has been accepted by the receiving server.
Unlike synchronous bounces, which occur immediately if there is an issue with the email address or server, asynchronous bounces can occur at any time, even weeks after the initial delivery attempt.
Asynchronous bounces can present operational problems, such as backscatter. Backscatter occurs when an asynchronous bounce is sent to the email address listed in the “envelope from” or “bounce address” of the original email, even if that address is not the actual sender of the email. This can result in innocent third parties receiving bounce messages for emails they did not send, or even being falsely accused of sending spam.
To avoid the issues associated with asynchronous bounces and backscatter, it is recommended that mail servers perform proper filtering and delivery decision-making while the sending mail server is still connected. This can help to safely reject unwanted email and prevent the need for asynchronous bounces.
Asynchronous bounces are commonly used in email marketing campaigns to notify senders of bounced emails and failed delivery attempts. For example, if an email campaign is sent to a large mailing list, some emails may bounce due to invalid email addresses or full inboxes. In these cases, an asynchronous bounce message can be sent to the sender to notify them of the delivery failure and provide information on why the email was bounced.
Overall, asynchronous bounces can be a useful tool for notifying senders of bounced emails and failed delivery attempts, but they can also present operational problems such as backscatter. By implementing proper filtering and delivery decision-making, mail servers can help to prevent these issues and ensure that emails are delivered safely and efficiently.
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