Why messages bounce back

There are many reasons why a message may have bounced back. If you’re not sure why, take a look at the bounce message, as 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 bounce error you're seeing does not sound similar to any of these errors listed, please contact our support team and include the full bounce message.

DKIM and/or SPF needs to be set up

If you are sending an email from a custom domain and you receive a bounce message, or the recipient says your message landed in their Spam folder, a good place to start is to check that your custom domain has DKIM and SPF set up correctly. 

DKIM and SPF are email authentication standards that confirm the sender is authorized to send from the domain and the message hasn’t been changed. Both need to be correctly set up in order to prevent outgoing messages from going to a recipient's spam folder or getting bounced.

To check your domain's DKIM and SPF, go to Settings → Domains and click the Recheck DNS button next to the domain. If your domain shows a warning that says DKIM and/or SPF records are needed, click Edit for further instructions on how to set this up. 

You’ll need to add these records at your DNS host. Usually, this will be the service where you bought your domain. If you’re unsure of where your DNS is hosted, contact our support team and they’ll be able to help you figure it out.

Content rejected

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 with an attachment is incorrectly flagged as a possible virus, ask the sender to zip the file before sending.

Access denied

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.

This error can also occur if you've misconfigured your email client. The bounce message should contain some contextual information.

Blocked using xbl.spamhaus.org

If the error received was blocked using xbl.spamhaus.org or a similar message, then the message was rejected because the sender was using a sending email system that has been detected as compromised in some way or sending excessive spam. 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. Bounce messages will indicate which server is causing the problem. Requesting removal from a global blacklist like Spamhaus is the responsibility of the owner of the domain that has been blacklisted, and they will need to contact that blacklist directly.

An email with an attachment bounced back

Fastmail rejects emails with certain attachments from being sent or received 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, appearing as a harmless text file when it's really an executable program. This is a common trick used to send viruses.
  • All .pif, .scr, .cpl and .com files: There is little reason to be sending these files, and they are commonly used to send viruses. 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 behavior when the same message arrives from different sources, as only the first one will actually be delivered.

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