Why do I get all these Failed Delivery Emails in my Inbox to people I don’t know?

Do you ever get an influx of emails, that say “Failed Message Delivery” or “Message Undeliverable” that are being sent to someone you have never heard of before, and that you know that you have never sent an email to?  No, it doesn’t mean that someone has hijacked your email, and is sending out tons of fake emails.  Believe it or not, these failed delivery messages are technically valid emails.  What these emails are known as is Non-Delivery Reports (NDR).  They are important parts of all email systems, and what they do is tell a sending email account, that a message that they have sent did not successfully reach its recipient.  But why are you getting these NDRs from email accounts that you have never sent to?

The answer to that, is spam.  A withstanding issue with email transportation is that the SMTP protocol (what is responsible for sending email) does not support authentication of the sender’s email address.  I will not go into details on how they do it, but to make a long story short, this flaw in email makes it possible for spammers to send email out, pretending to be from another valid email address.  So basically what happens is, a spammer sends out an email with spam to an address that may or may not exist, pretending to be sending as your email address.  When the email they are sending to does not exist, an NDR is generated, and sent back to the email address that it came from.  Only instead of going back to the spammer, it goes to your email that was faked as the original sender.  Normally this wouldn’t be a huge deal if you only got 1 or 2 of these NDR emails; however most of the time spammers send out hundreds of thousands of emails at once, and your inbox could then get flooded with NDR messages.

So what can be done to block these messages?  Normally, you can just put a content filter on to block certain words or phrases in emails.  However, in this situation, creating a filter on an NDR, while it make block the fraudulent NDR messages, will also inevitably block and legitimate NDRs that are generated from you actually emailing someone and the email not reaching them.  Most of the time there is not much that can be done to stop these emails from getting through without also block legitimate messages, which is why a lot of spammers use the technique.  While there are some expensive third party applications that an administrator can run on their email server to help prevent fraudulent NDR messages, they do not always work perfectly.  Luckily, this type of spam attack does not last long, and is usually completely over within 24 hours.  While it can be an inconvenience, as long as you’ve identified what the issue is, and take care of it appropriately, there is no real danger to worry about.