When you send an email to someone and it bounces back, the server that bounced it back to you will usually include a message in the email called an "SMTP Reply" (which is meant to explain why the server bounced it back to you). That information is located in the header of the bounced message.

When MailChimp gets a bounced message, we try to read its SMTP reply, then categorize the email as a "soft" or "hard" bounce, so you don't have to do a thing. But the system isn't perfect. Some email servers don't send bounces with properly formatted SMTP replies that use standard codes. In those cases, MailChimp has to make an "educated guess" as to how to categorize that bounced message. If you want to know specifically why a message bounced, or if you feel MailChimp categorized a bounce improperly, you'll want to read its SMTP reply.

MailChimp stores all bounced messages so you can read these SMTP replies, and determine exactly why each message bounced.

  1. To view your bounces go to the Reports page in MailChimp and click on the name of the campaign that you're interested in.
  2. Underneath the open and click rate graphs, you'll see the number of bounces. If the number of bounces is greater than zero, the number will be a link to your bounce reports.
    the number of bounces is a link!
  3. The next page will be "who bounced" on your subscriber activity page. We'll show you how many soft bounces, hard bounces and a list of subscribers whose email addresses bounced on this send.
    subscriber activity, bounces
  4. Click on "bounce reason" to read your bounce headers*.

You'll have to visually scan for the SMTP reply (not all servers place this data in the same place, nor do they format it consistently, nor must they adhere to any standards. It's a very manual process, unfortunately). Below is an example of what to look for. Look at the Diagnostic Code, rather than "Status" notation.

Example SMTP reply in email header

We also factor engagement activity into our bounce cleaning decisions (read about how MailChimp measures engagement).

For example, if we send an email and a receiving server tells us that a recipient “does not exist,” but we have open and click activity in the last 45 days to prove otherwise, we’re not going to blindly clean that recipient from the list. We know they exist, and we know their account works, so we’re going to give them a few more chances than we normally do. If, however, we see that there’s very little (or no) activity by that recipient, we clean them under the same rules we’ve used in the past.

We can't get into specifics about how many stars justifies a “clean vs. a keep,” or exactly how many chances we give hard and soft bounces. The algorithm will surely be adjusted and tweaked over time.

*The full text accompanying each bounce are removed from campaigns after 30 days.

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