Monthly Archives: October 2011

Other Email Bounce Backs and Their Meanings

Bounce backs and error codes for email can be very mysterious and misleading. To help better understand them,we have started a new series on the blog dedicated to demystifying these occurrences. To read all of the blogs in this series please follow this link.

We’ve already covered a few of the different types of bounce backs but we haven’t really even scratched the surface! Email error messages can be broken down into three groups: User Error Messages, Domain Error Messages and Anti-Spam Error Messages.

User Error Messages
These are typically local issues with the user’s email account or email client. They include mailbox is full, message exceeds size limit (attachment size), and user unknown, mailbox unavailable or invalid recipients.

User Unknown
Probably the most common bounce back we see is the user unknown, mailbox unavailable and invalid recipients. Simply put the email address you are attempting to email, doesn’t exist. Typically these are due to misspellings of the user name or domain.

<>: host said:
550 5.1.1 <> is not a valid mailbox<>: Sorry, no mailbox here by that name. (#5.1.1)<>: host said:
550 Invalid recipient

If you receive a similar bounce back, confirm the spelling of the entire email address and resend if necessary. If you have confirmed the spelling then you can try and contact the user via an alternate method. Sometimes users don’t know they are having an issue!

Mailbox is full
Most mail systems have a limit on how much email is allowed to remain on the server for each individual user. If that limit is reached the server will not allow them to accept any new mail.

<>: User is over the quota. You can try again later.

<>: host said:
552 <>… Mailbox is full

Since this is a local issue with the user’s mailbox, their system administrator will need to either make room for new mail or increase their storage allocation. Typically you can resend your message a bit later as this type of problem is easily resolved. Keep in mind, that if you continue to receive the error that may mean that the account is no longer being monitored.

Message Exceeds Size Limit
This error indicates that the size of the message including email headers, message content and attachments exceed the domain per message size limit. Typically most mail servers only allow 5-10mb per message as a default. Email was never meant to be a way to send large attachments, it is instead recommended to use a 3rd party sending service, FTP server or another alternate method.

<>: host said:
552 message size exceeds maximum message size<>: host said:
552 Message size exceeds fixed maximum message size

Domain Error Messages
These type of errors usually have to do with a domains registrar expiration or DNS issues. If these issues occur you may receive a bounce back indicating a Connection Timed Out or Domain Not Found.

Connection Timed Out
A “connection refused,” or “connection timed out” error usually indicates a message sending issue. This could be due to a high volume of messages, an external spam attack on the server or an internal setup problem. Typically these are resolved rather quickly by the server automatically so you can resend your message a bit later.

<>: connect to Connection Timed Out

Domain Not Found
If you receive an error indicating that domain could not be found or no DNS record exist, this means that the domain doesn’t exist. This may be a temporary issue where the domain has expired or it could mean there is an MX Record issue with their DNS.

<>: Name service error for domain
Host not found, try again

Anti-Spam Error Messages
Everyone hates to get spam and there are hundreds of ways to try and stop it. One way that administrators use is to issue bounce backs if they believe a message is spam.  Often times, these are custom created bounce-backs so the error codes can vary, but the message is all the same. Stop sending spam!

NOTE: We do not advise using bounce backs to combat spam. This form of anti-spam may actually allow your users to get MORE spam. Instead we would highly recommend that anyone running a Business Email Server invest in an advanced heuristic spam, virus and phishing protection service, with controls featured in modern anti-spam and anti-virus products and services such as our own Spam and Virus Business Email Protection. We also include these services in our Email Hosting services.

<>: connect to 550 Connection refused – we hate spammers!

<>:host said: 554 Denied

<>:host said: 552 spam source blocked

If you are receiving these types of bounce backs, we would highly recommend checking if your mail-serer IP Address is on a Blacklist. While your mail may be legitimate to you, others may not see it that way. If your company gets Blacklisted, it could cause major trouble for your business and slow down communication with your current customers or prospects and in general, the outside world.

There are many reasons an IP Address may end up on a Blacklist.  More often that not it’s because the administrators controlling it have not taken appropriate steps to secure their email infrastructure or the network has workstations that have been compromised by spammers, hackers, or virus propagators.

Bounce messages are all very different and may contain different languages but diagnosing the error code can help you understand it.  A good rule of thumb is to ensure that your messages are clean, simple and desirable.  This will go a long way to making sure your message reaches the recipient.

Taking the time to ensure that your messages get delivered is incredibly important.  Take the extra step and get advanced, real-time monitoring of your server against blacklists, as well as availability and performance. Please visit our website to learn more – MxWatch Monitoring – Email | Website | Network.

Additional Resources
400/500 Email Bounce Back Errors Explained
How to Read Email Bounce Backs and Errors
What Blackslists Are & How MxToolBox Helps

Temporary relocation of the ‘Manage this domain’ link for Administrators in Google Apps Gmail

from Google Apps:

UPDATED 11/3: If an administrator account switches to Gmail’s new look, the ‘Manage this domain’ link moves from its current location, to the Settings gear menu in the upper right of the Gmail inbox. More Details.

Google Apps administrators have a link available in their Gmail inbox which allows them to easily access their administrator control panel. This link is an alternative to logging in at where is the name of the admin’s domain.

This ‘Manage this domain’ link is temporarily moving from its current location at the top right of the inbox, to the bottom right corner of the inbox, just underneath ‘Last account activity’. This relocation is related to the new look being rolled out to Google Apps services and we’ll provide more updates on its final location when available.

Release track:

Editions included:
Google Apps, Google Apps for Business, Government and Education

Languages included:
All languages supported by Gmail

How to access what’s new:
In an administrator’s Gmail inbox, scroll down and the ‘Manage this domain’ link is now located on the bottom right side.

For more information:

SysQuadrat Blacklist Removed from Our Blacklist Tool

We recently had added a new Blacklist to our tool called SysQuadrat with the cooperation of the list manager. Unfortunately the RBL provider didn’t realize the amount of traffic that our tool would be sending him, so his list got a bit more than bargained for. The RBL Provider also let us know that his list is only for Paid Users. At this time we have removed the SysQuadrat Blacklist from our tool.

For more information on what Blacklists are and how MxToolBox can help, read this blog post.

MxToolBox is not affiliated with any Blacklists, we just provide a public tool that can be utilized to see if you are on a Blacklist. If you are on a Blacklist, rest assured as we do offer Blacklist Protection! For more information please feel free to visit our website or email us at

We’re always looking for new Blacklists to add, so if you want to suggest one to us, please send an email to

For information about other Blacklists that have shut down or Blacklists that are having problems, view this forum post.

October Newsletter – Demystifying Email Bounce Backs and Errors


Last month we started a series of posts on our Blog covering Email Bounce Backs & Errors. These errors can be incredibly confusing and misleading, so we wanted to try and demystify these problems. Read on for a few of the most recent articles that cover a wide spectrum of email problems related to bounce backs and associated error messages.


Non-Delivery Report (NDR) Spam or Backscatter Spam

Monday, September 19th, 2011

In our continuing blog series about bounce backs and error codes we wanted to talk about NDR Spam or Backscatter Spam. As we all know, spammers are tricky devils and they spend the majority of their time learning to adapt and circumvent email defense systems. One example that demonstrates the type of adaptability that Email Security professionals have to deal with is Backscatter spam. As an operator of a legitimate email server […]


Posted in Bounce Backs, Knowledgebase| 1 Comment »

Bounce Backs: Denied For Spam, Message Rejected, Spam Source Blocked, What Does it Mean?

Wednesday, September 14th, 2011

Have you ever received a bounce back that refers to your message as being blocked because it was considered spam? While the actual language of the bounce back or error message may vary if the error code is a 500 error, that does mean the message could not be delivered to the recipient (400/500 Email Bounce Back Errors Explained). In this particular case, we are referring to bounce backs that reference messages as being denied due to spam or IP reputation […]


Posted in Bounce Backs, Knowledgebase | 1 Comment »

400/500 Email Bounce Back Errors Explained

Tuesday, August 30th, 2011

Bounce backs and error codes might look like they need to be deciphered with a secret decoder ring. We are going to try here to shed some light on them so you can crack them open and extract the goodness within. So let’s work together to understand the the two most common types of bounce backs. When a bounce back message is generated, the mail server that issued it is attempting to let you know there was a problem with sending the message and give you some information so you have an idea of what […]


Posted in Bounce Backs | 1 Comment »

550-”5.7.1 Message rejected as spam by Content Filtering.” – Intelligent Mail Filtering with Exchange

Friday, August 19th, 2011

This is an error that our customers run into pretty often here are MxToolBox so we thought we would help explain the cause and provide some solutions to remedy it. By default Microsoft either installs/or recommends that the Intelligent Message Filtering (IMF) service be enabled on all installations of […]


Posted in Bounce Backs, Knowledgebase | 1 Comment »

‘4.3.1 Insufficient system resources’ – Back Pressure Feature Exchange 2007

Monday, May 9th, 2011

We see this type of error pretty often here at MxToolbox and so we wanted to post about it here. Customers Exchange servers can mistakenly react to normal mail flow and cause a disruption in service. The error that is posed is ‘4.3.1 Insufficient system resources’ […]


Posted in Bounce Backs, Knowledgebase | Edit | 1 Comment »

How to Read Email Bounce Backs and Errors

Thursday, May 5th, 2011

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 […]


Posted in Bounce Backs, Knowledgebase | 2 Comments »


We have been supporting our Business Email Products (Spam & Virus Filtering, Emergency Mail and Hosted Email) for years and we take pride in sharing all the knowledge we can with our customers and non-customers. We hope that these articles help you maximize the impact of your efforts by utilizing MxToolBox, our Free Tools, Server Monitoring and Business Email Products to their fullest potential.

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