Category Archives: Bounce Backs

How to Read Email Bounce Backs and Errors

Bounce backs and error codes for email can be very mysterious and misleading. To help with that we have started a new series on the blog dedicated to demystifying these occurrences. To read all of the blogs in this series, go here.

To kick off our series on demystifying and understanding email bounce backs and errors, we thought it would benefit everyone to go over how to read a bounce back. Some bounce backs are very cryptic and full of codes and numbers. How are you supposed to figure it out? Let’s break down a typical bounce back:

  1. The top part of this message is the actual bounce back. This is the “meat” of what you need to identify.  Sometimes bounces include lots of numbers and codes; ignore all that and find the string that references the 400 or 500 number.  (What’s the difference between a 400 and a 500 error?).  In this case the error is ‘550 No such user’. Since this account doesn’t exist at the message was bounced by the recipient server.
  2. The second half of the bounce is the email headers. Keep in mind that not all bounce backs include this information, however, most do. This information is really helpful as it contains the Sender, Recipient, Date, Time and Subject, as well as server hops. If you are unable to figure out the issue, make sure you send the complete bounce back including the email headers to your IT administrator.  All of this information is critical in understanding a bounce back. If you need help reading headers, try our free tool, the Header Analyzer. It makes the email header a bit easier to read.

Microsoft Exchange
As with all things Exchange, they have their own way of doing things. Exchange bounces include a top header section; however, we tend to ignore that section as it has very little helpful information.  Remember to focus on the “Technical details” or the “Diagnostic Information for administrators;” as this is the “meat” of the data you need to analyze.   You may also notice that Exchange bounces include two conflicting “who rejected your message” statements. The second one labeled “Generating Server” is generally the server that issued the bounce.

Remember that knowledge is power!  We at MxToolBox are constantly educating ourselves about all the different bounce backs that exist.  Also keep in mind that with some Vendors and ISPs you have the ability to create custom bounce back errors…so you always have to be on your toes!

If this is a bit overwhelming or you don’t want to mess with understanding bounce backs or error codes, don’t worry.  It can take years of experience to feel comfortable reading and deciphering this information. We understand that you just want your email to work!  Implementing one of our Managed Business Email Products such as Spam and Virus Filtering or Hosted Email can help alleviate these issues and put someone in your back pocket to help understand when these problems occur.

Blocking Non-Delivery Report (NDR) spam with HTML Attachments

We have posted a few helpful hints for users that are experiencing problems with Password Reset requests, UPS, Western Union, Youtube and other forms of spam. However, it looks like the spammers are altering the message to adapt to the changes that Postini and other vendors are making, so more updates to the filters are expected.
We are recommending that a temporary custom attachment filter to block all messages with a .html attachment is enabled within Postini. NOTE: If this filter is applied, it will block any legitimate message with that type of attachment. See below for the steps to enable the filter and the recommended settings:

Attachment Manager Filter Steps
  1. Access the customer’s Postini User Org and enable the Inbound Attachment Manager.
  2. To build a custom filter for blocking .html attachments, select Filter and follow the image below:

  3. We highly recommend enabling ‘Scan inside compressed file types’ and ‘Enable binary scanning’ as this may also help with any future evolutions.
  4. Be sure to add ‘html’ under 2. Custom Filter Types to either User Quarantine (in case of false positives) or under Quarantine Redirect.
  5. Click Save and the filter is applied.

MxToolbox has partnered with WebRoot to offer Web Filtering to protect your network from attacks through the web browser. For more details on the protection that this program can offer, go here.

What does the Bounce Message – “Unable to Relay” Mean?

If your customers are receiving an error message like below, there are 2 settings that may need to be adjusted.

‘’ on 9/15/2006 11:11 AM
550 5.7.1 Unable to relay for

Configure your Exchange Server to accept mail anonymously for your domain

The directions below are for Exchange 2007, but most mail servers should have similar settings.
  1. Please confirm that your Send Connector has Anonymous Users allowed under Permission Groups.
  2. Open your Exchange Management Console and access Server Configuration > Hub Transport > Receive Connectors

  3. Right Click on the Default (or any other Receive Connector your company uses) and choose Properties.
  4. Select the Permission Groups tab and ensure that Anonymous Users is checked.

    NOTE: This does NOT allow anonymous users to send mail through your server, this would configure the server as an Open Relay; that would be bad. This allows anonymous users to  have access to the Receive Connector so they can send mail addressed to your domain(s).
Configure your Email Client to authenticate when it connects to your SMTP server
To resolve this issue please adjust their email client to require ‘My Outgoing Server SMTP Requires Authentication’. The directions below are for Outlook 2007, but most mail clients should have similar settings.
  1. Open Outlook
  2. Go to Tools > Email Account > Change
  3. Click More Settings > Select Outgoing Server tab and ensure that check box next to My Outgoing Server SMTP Requires authentication is enabled.
  4. Click Ok and Next and Finish.