When you send an email to someone and it bounces back, the server that bounced it back to you will usually include a little 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" bounceback, 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.

To view the full headers in a bounced message - click on the reports icon to the far right of your Campaign Title, on that page click on the "bounce" link at the top left ish on the reports page. The bounced email addresses will appear in a list on this page. 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.