What are Out of Band Bounces (OBB)?
In order to explain what Out Of Band bounces (or OBB, sometimes also called Asynchronous Bounces) are, we first need to understand how a recipient server accepts email.
In most cases, a recipient server will either accept or reject an email as it comes in. In those cases it will look at the email, and issue a code 250, which means the message is accepted. If it’s going to bounce, it will issue a code in the 400’s or 500’s, such as a 550.
However, there are some recipient servers that are configured to accept incoming emails in a holding state, and determine later whether the emails are going to be sent to the inbox, or rejected and bounced. Thus, these bounces can take a while to appear, and in this case you may see both an acceptance of the email, and later a bounce. This is why we call them Out of Band, because the bounce would arrive outside of the normal email server transaction.
We have recently made changes to our processing of OOB bounces, and so you may see a slight increase in bouncing. If so, this should even out fairly quickly. However, depending on the rejection error given by the recipient server, OOB bounces may not have the proper rejection message, so if you notice an email address being OOB bounced consistently, you may wish to put them on your suppression list.