Why messages bounce back

There are many reasons why a message may have bounced back. If you’re unsure why, take a look at the bounce message — it should include a description of the error. Below are some common reasons messages bounce and explanations of the errors that can occur.

If the error received was blocked using xbl.spamhaus.org or similar such message, then the message was rejected because the sender was using a sending email system that's been detected as compromised in some way. Most commercial email sites, including ours, block messages from insecure servers because almost all messages originating from them are spam. These are global blacklists, and are not controlled by FastMail.

Links contained in bounce messages tell the sender's email administrator which server is causing the problem. Contacting the sender's email administrator is helpful because all emails sent from that provider may be getting bounced or silently ignored. To get off of a global blacklist like Spamhaus, they'll need to contact that blacklist directly.

If the error received was content rejected, then the message was rejected because it either contained a probable virus, or it is a message previously reported as being spam. If a message is flagged as a possible virus incorrectly, ask the sender to zip it before sending.

If the error received was access denied, the sender or recipient has been blocked by the server. It's possible that they have been reported as a spammer, or they may have been blocked by our mail abuse systems. Often, though, this error can occur if you've misconfigured your email client. The bounce message should contain some contextual information.

An email with an attachment bounced back

We reject emails with certain attachments due to security reasons. The blocked attachments are:

  • Double-extension files (e.g. something.doc.exe): Many Windows systems hide all but the first file extension by default, so a file called something.txt.exe will appear as something.txt, apparently a harmless text file when it's really an executable program. This is a common trick viruses use to try and spread themselves.
  • All .pif, .scr, .cpl and .com files: There is little reason to be sending these files, and they are commonly used by viruses as a way to spread. If you need to send one of these files, zip it first, then send the zip file.

The same email from multiple sources fails to arrive

Most messages have a unique identifier called the "message ID". When a message arrives at an account, the message id is noted. If a second message with the same id arrives, it is ignored and silently discarded. This is used to stop infinite mail loops overloading your account or our system. However, it can result in odd behaviour when the same message arrives from different sources, as only the first one will actually be delivered.

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