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Email Bouncebacks Demystified

What’s in a name (header)

About once a week I receive a question from a user asking why their email did not deliver, often they think there is something wrong with our server. With a little bit of knowledge transfer, a user should be able to identify the reason for themselves. However, most are intimated by the “alien language” in a typical email header, no matter how user friendly we make the error message. With this post hopefully those pesky email bouncebacks will be demystifed.

Email Header

MIME-Version: 1.0
Received: by with HTTP; Mon, 6 Mar 2017 11:55:26 -0800 (PST)
X-Originating-IP: []
Date: Mon, 6 Mar 2017 14:55:26 -0500
Message-ID: <>
Subject: This is a header
From: Matt Pieper <>
To: Matt Pieper <>
Content-Type: multipart/alternative; boundary=001a11406d80cb384e054a154671
If this looks familiar, you have probably received a bounced back email. If you are like most users have widened your eyes in confusion. I certainly cannot blame you , as there is a lot to take in. However, for the average person you only need to check a few things to see what happened to your message.
Most of the information in the header you will recognize from sending the email:
  1. Date
  2. Subject
  3. From
  4. To
There are other items that you won’t recognize (and that’s okay):
  1. X-MS-Has-Attach
  2. Accept-Language
  3. MIME-Version
  4. Message-ID
  5. Others
These are items that the servers need to route the message, and display them in your email client.

What do I need to find?

What we are looking for is specific information on why your message did not deliver. This information is usually found near ‘Diagnostic information for administrators’ or ‘Delivery has failed to these recipients or groups:’ and should look something like this:
Remote Server returned ‘< #5.0.0 smtp; 552 5.3.4 Message size exceeds fixed maximum message size>’
This line is telling us that our email is too big (most likely due to attachments), and that we should reduce the size and send again. This is because most servers limit attachments to 25MB or less to reduce storage and/or improve bandwidth. How did I know it was the receiving server? Due to the phrase ‘Remote Server returned’.

Common Codes

Error codes wording can vary by server, but here are some common reasons, but the numbers are usually the same:
  • 550 – user not available. Could be a typo in the email address or user is no longer with the organization.
  • 552 – message is too large. Try removing attachments and try again.
  • 553 – recipient not found. Similar to 550.
  • 5.3.1 – mail system full. Less common due to cloud based email systems, but on-premise Exchange servers can see this a lot.
Most email providers are realizing that the average user is not a network administrator and are making strides to use plain English to describe a failure reason. The biggest thing to remember when receiving a bounced email is to take a deep breath, and scroll down. The answer is usually in plain sight!

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