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.

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.

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.

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.

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.

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.

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