Why messages bounce back

If you send an email that is unable to be delivered, you will receive a message letting you know that the message is undeliverable, or "bounced." There are many reasons why an email could bounce, and bounces can happen on either the sender's end or the recipient's end.

Bounce messages contain errors that explain why the message could not be delivered, though these errors can often be vague or confusing. On this page, we'll go over some of the most common bounce message errors, as well as some general guidance and tips on how to read and understand bounce errors.

If you're having trouble understanding why your message bounced, or if the bounce error provided seems inaccurate, please contact our support team so they can look into it for you.

Common Fastmail bounce errors

If the bounce message you received came from Fastmail, you may see one of the following errors (though this is not an exhaustive list):

550 5.1.1 Recipient address rejected: User unknown in virtual mailbox table

This error message indicates that there does not seem to be a user with this email address. In some cases, this error may also show as 550 5.1.1 Sorry, user address unknown.

If you get this error message, the first thing to do is check the email address to make sure you did not enter any spelling mistakes or typos.

If you are sure that the spelling you have is correct, you may want to contact the recipient through an alternate channel, if possible. You can let them know that you're seeing this bounce error, as it could indicate that their email account is inactive or is not successfully receiving mail.

551 5.7.1 Not authorised to send from this header address

This error message indicates that there is an issue with the email address you're using to send the message. You will want to go to your Settings → My email addresses screen and find the email address you used to send this message. If the email address has an Unverified status next to it, you will need to set up authenticated sending before you can send mail from this address.

535 5.7.8 Error: authentication failed: authentication failure

This error message also indicates that there is an issue with the email address you're using to send the message. In this case, you likely have an external address set up with authenticated sending, but the password needs to be updated.

You will want to go to your Settings → My email addresses screen, click the Edit button for this address, and click Use authenticated SMTP when sending. From there, you can use the Check server settings and Update password buttons to check if the password is correct and update if needed. 

Host or domain name not found

This error message indicates that there is an issue with the recipient's domain.

You may want to contact the recipient through an alternate channel, if possible, and let them know that you're seeing this bounce error. The recipient (or whoever owns the domain) will need to address the issue at their domain registrar or host.

The full error message will provide some details as to the specific issue that is occurring:

Host or domain name not found. Name service error for name=<domain> type=<DNS record>: Host not found, try again

The <domain> portion will list their domain name, and the <DNS record> portion will list the specific type of DNS record causing the issue (e.g. MX, NS, or A). If the recipient is not sure how to fix this issue, their domain registrar or host should be able to provide assistance.

Common bounce errors from other email providers

Every email provider writes their bounce errors differently, but certain issues are often described using similar language. 

Invalid recipient, user not found, etc.

This type of error message indicates that the recipient's servers can't find a user with this email address. Check the email address to make sure you did not enter any spelling mistakes or typos.

If you are sure that the spelling you have is correct, you may want to contact the recipient through an alternate channel, if possible. You can let them know that you're seeing this bounce error, as it could indicate that their email account is inactive or is not successfully receiving mail.

Mailbox quota exceeded, mailbox storage full, etc.

This type of error message indicates that the recipient does not have any storage space available, so their account is unable to accept new messages. You may want to contact the recipient through an alternate channel, if possible, and let them know that they will need to clean their inbox to free up some space.

If the error message you receive includes a 4xx error code, this means that the email system will continue making attempts to deliver the message for a short amount of time. If the recipient is able to free up space within that time period, the message will then be delivered. If the error message includes a 5xx error code, this means that the message is considered undeliverable and the email system will not make further attempts to deliver it. (For more details on how the 4xx and 5xx errors work, see our Hard bounce vs. soft bounce section below.)

Spam message rejected, suspicious content detected, etc.

This type of error message indicates that the recipient's servers suspect that the message is spam. This could be due to a number of reasons, and depends on how strict the server's spam filters are.

If the error message mentions "suspicious content" or something similar, the issue could be due to a link, attachment, or particular phrase (e.g. mentions of "billing," "Bitcoin," etc.) in the email's content. It may help (even just for the purposes of troubleshooting) to reword certain phrases, or remove any links or attachments, then try sending again.

Connection refused, connection lost, etc.

This type of error message indicates that we're having trouble connecting to the recipient's servers. This kind of issue can be caused by a lot of different things, such as an issue with a mail client, a strict firewall, an issue with the recipient's domain, or an issue where we're blocked or blacklisted by the email provider. If you're getting this bounce error and you're not sure why, our support team can help with troubleshooting.

How to read bounce errors

If you're having trouble understanding a bounce error, you can always contact our support team for assistance. If you're interested in learning more about how to read and understand bounce errors, we've listed some basic information and tips below.

Hard bounce vs. soft bounce

Many bounce error messages will include three digit codes starting with a 4 or a 5 (e.g., the 550 in 550 5.1.1 Recipient address rejected). These codes indicate if your message has been hard bounced or soft bounced.

If the code starts with 5 (also referred to as a 5xx code), this means that the message has been hard bounced. A hard bounce occurs if the mail system permanently rejects the message. If this happens, the only way for the message to get delivered is if you manually re-send it after the issue causing the bounce has been resolved.

If the code starts with 4 (also referred to as a 4xx code), this means that the message has been soft bounced. A soft bounce occurs if the mail system believes that the issue causing the bounce may be temporary, and decides to continue making attempts to deliver the message for a short amount of time (often 1 day or so). If the issue is resolved within that time period, the message will be delivered. If the issue is not resolved, you will then receive a hard bounce message with a 5xx code.

Check for links

Some email providers will include links to help pages in their bounce messages, which can provide some general background information on why the bounce occurred. (Beware of phishing — before clicking any links in a bounce message, be sure that it is a legitimate bounce for a message that you know you sent.)

I'm getting bounce errors for messages I didn't send

If you're receiving bounce messages for emails that you don't recall sending, you may be experiencing backscatter. This can happen if a spammer spoofs your email address and sends spam that is undeliverable, which then causes the bounces to be sent to you. For more detailed information on how to handle backscatter, please see our Backscatter help page.

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