Earlier today I needed to find out if a bounced email occurred synchronously or asynchronously. During the course of my research, the thought occurred to me that the difference between synchronous and asynchronous bounces is probably fairly unknown among email marketers, but knowing the difference would probably come in handy for them.
In order to explain the difference between a synchronous bounce and an asynchronous bounce, I’ll first need to explain the basics of how email servers communicate with one another while sending and receiving emails. First, the sending server contacts the receiving server and says “hey, I am server XYZ and I have an email to deliver to you.” The receiving server then checks to see if (a) it wants to receive email from the sending server, and (b) if it can deliver the email to the specified recipient.
At this point, the receiving mail server can do a number of things:
- Accept the email message and deliver it to the recipient.
- Temporarily refuse to accept connections from the sending server.
- Permanently refuse to accept connections from the sending server.
- Respond that the recipient does not exist and refuse the connection.
If any of the above responses, except number 1, are received by the sending server, the attempt to send that email is aborted and it is considered a Synchronous Bounce.
If the receiving server responds that it will accept the email message, then the sending server attempts to send the message. If the process of sending the email is aborted or interrupted for some reason, this may also result in a Synchronous Bounce, depending on the settings of the sending server (some of them will retry for a certain number of times for various types of interruptions).
If the receiving server indicates that it will accept the incoming email, and the process of sending the email message completes without complications, the receiving server will respond that the entire email was accepted successfully and the sending emails records a successful transaction in its server logs (actually, all of this is recorded in the server logs).
If after a successful transaction, the sender (the actual person sending the email) receives an email message from the receiving server indicating that the email message was not delivered for some reason, this is known as an Asynchronous Bounce. You are probably familiar with this kind of email because most people have received them. These are the emails that say something like “Your message could not be delivered for the following reason:” and then have a bunch of seemingly garbled text that supposedly explains the cause the failure.
Most email marketing systems have a method of capturing the messages from these Asynchronous bounces and then marking the message as “bounced” in their metrics reporting. Because the receiving server initially responds that the email message was delivered, and then sends the non-delivery message at a later time, this lag can cause the email reporting metrics to change and make it seem that “delivered” emails change and mysteriously morph into “bounces.”
It is important for email marketers to have a rudimentary knowledge of the difference between Synchronous and Asynchronous bounces so that they can effectively maintain their lists and interpret the metrics associated with each email campaign. One reason that email systems may return an Asynchronous bounce message after initially accepting the email is because the email system may run the email message through some content filtering software and other tests to try and determine if the email is spam and should be rejected. If this is happening alot in your email campaigns, this may be something you want to check, so that you can ‘despamify‘ the language in your email, and/or otherwise figure out what it is about your email message that the email systems don’t like. Correcting this will help get your message into your recipients’ inboxes.
Hopefully, this information will help you become a better email marketer. If you have other technical questions about email marketing, feel free to post them in the comments, and I will do my best to provide answers. Thanks for reading.













