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What does my email bounce mean?

When a sent message can’t be delivered, it bounces or comes back with an error message (also called a bounce back) that describes the reason delivery failed. Read this error to understand what caused the issue and how to resolve it.

The bounce error will show in the received message. This is how an error might look in Outlook, though your error message could look different depending on the email provider rejecting the message. The area outlined in red is the reason for the error.
Tenant has exceeded threshold error message

Common reasons for email bounces

Rejected by the recipient

Errors that include: Mailbox unavailable, Undeliverable, Connection timed out, or similar messages.

Sometimes the person you’re trying to message can’t receive email, or their inbox or receiving folder might be full. This issue usually goes away on its own or you can try again later. If someone is trying to email you and gets one of these errors, you might need to update your MX records.

How you can fix this:

  • Check the recipient email address for typos.
  • Wait a few minutes and try resending the message.

Email was flagged as spam

Errors that include: Blocked, Denied, Access denied, Unacceptable Content, or similar messages.

Your email might have been considered spam by a spam filter or firewall. You can test if the content of your email is what’s causing the block by sending a plain-text email with no attachments to a single recipient. If that test message is received, the issue is likely with the original email’s content.

If an Internet Service Provider (ISP) is blocking your email, or your IP address, this is usually temporary, and your email should work again once your address stops getting spam complaints. You can also contact the recipient to add your email to their Safe Senders List (or Allow List).

How you can fix this:

  • Make sure all links go to valid destinations that are free of malware and phishing scams.
  • Only include valid, correctly spelled email addresses as recipients.
  • Reduce your number of recipients, especially recipients to Cc or Bcc.
  • Give recipients the option to opt in or out of your marketing emails. (Learn more about email marketing best practices.)
  • Contact the recipient and have them add your domain or email address to their Safe Senders List (also called Allow List).
  • Make sure your domain has SPF records. If your email and domain are with Filthy Rich Idea, we automatically do this for you.

 Note: If you’re not sending bulk quantities of email and you get this type of bounce error, your email account might have been compromised and a bad actor could be using it to send malicious spam. Follow these steps to unblock your Microsoft 365 account. We also recommend changing your email password and securing your account with multi-factor authentication (MFA).

Recipient email account doesn’t exist

Errors that include: Mailbox not found, Recipient not found, Invalid mailbox, User unknown, or similar messages.

The recipient’s email address might not exist anymore.

How you can fix this:

  • Check the recipient email address for typos, including punctuation at the end of the address or spaces before or after the address.
  • Contact the recipient using a different method to find out if they changed their email address.

Related step

More info

  • To send and receive messages, your email account must be set up correctly.
  • If you can’t send email but aren’t getting a bounce error, make sure you haven’t exceeded your email limits.
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