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

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.

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. The bounce back message itself will help identify why the message may have been denied (How to Read Email Bounce Backs and Errors); content of the message, unsolicited commercial email, or the Internet Service Provider (ISP) or email provider has a sending IP Address reputation problem (Blacklist).

Example Bounces
551 Denied for Spam
554 Service unavailable; Client host [<hostname>] blocked using Barracuda Reputation
554 Your access to this mail system has been rejected due to the sending MTA’s poor reputation
554 Denied (Mode: normal)
550 5.7.1 Message rejected as spam by Content Filtering
571 spam source blocked – psmtp

Message Content
The subject line and content of an email message are incredibly important! These two components can often lead to a spam filter flagging a message as spam and either placing the message in the recipient’s Junk Folder or worse, sending the message into a black hole never to be seen. We highly recommend taking extra time to ensure that your message has valuable content that someone would want to read and doesn’t sound too “spammy” or “salesy.”  This may seem simple, but it is amazing how often this is overlooked.

Another critical element to consider when constructing your message is that most larger Email Service Providers are using human influence in their spam scoring. This human touch is important to consider as users finally have the power to influence spam filtering. When users mark a message as spam some providers use this data to flag similar emails as spam and may add your email address, domain, or IP to a Blocked List. Sometimes the message is in fact a legitimate mailing that was requested by the end user but in the end if the recipient does not want the message in their inbox, they will often mark it as spam (The Search for the Perfect Spam Filter – October Newsletter).

Email Signatures
We have been seeing more and more issues with email signatures causing messages to be blocked. Just like the content of your message, be sure to keep your signature simple and free of extraneous information. For instance if you are recommending an outside company’s URL, make sure they are not Blacklisted and that they don’t have domain reputation problems. If certain messages are not getting through your recipient’s spam filter, make sure your signature is as clean as possible. You may also consider removing any images in your signature as well as that is a tactic that spammers will often use.

Explicit Blocked List
Another way that you could receive this bounce back message is if your email address or domain has been added to an explicit block list. This means that someone adjusted their spam filters to specifically block messages from your email address or domain. Unfortunately there is not much you can do in this case other than reach out to the recipient by other means to ask if they will consider removing the block. However, if they took the time to adjust the filters they usually have a reason for it.

If your company gets Blacklisted it could cause major trouble for your business and slow down your communication with current customers, prospects and in general, the outside world. A Blacklist, also known as a Real Time Blacklist (RBLs) is a list of problematic IP Addresses that are compiled by organizations monitoring spam on the Internet. There are many such organizations ranging from one person tinkering in their free time to large multinational corporations. MxToolBox provides a Free Blacklist Lookup Tool that will check an IP Address aggainst over 100 different blacklists. We do not control nor are we affiliated with any of the organizations running the lists; the tool simply performs a search against each list and aggregates the data into one result. Without such a tool in place, you would need to go to the website for each list and manually search for yourself. 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 language but if they contain wording like Denied, Spam, and the like, it means they were more than likely blocked due to one of the issues listed above. Ensuring that your messages are clean, simple and desirable to the recipient 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

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

  1. Sally Thompson

    This is a very helpful post, our messages were going into the spam folder of the people we were sending to and we had no idea why! It turns out we had a web site in our signature that was causing the problem. Once we removed that, no more problems!

    We also use your Blacklist Tool and have peace of mind that our servers is being monitored. Thank you!

  2. Pingback: October Newsletter – Demystifying Email Bounce Backs and Errors

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