Send & Receive Issues

Fix Emails Returned as Undeliverable to Cox.net Addresses

Maira Mehtab Sep 27, 2026 869 views

Question details

The user needs to understand why emails sent to Cox.net addresses are bouncing back as undeliverable and how to address the rejection.

Product: Email Service
Device & OS: not provided
Scenario: Sending emails to recipients who use Cox.net email addresses.
Observed behavior: Messages are rejected and returned to the sender by Cloudmark due to filtering, poor sender reputation, or strict email rejection policies.

Before you start

Verify that you have typed the recipient's Cox.net email address correctly and ensure your email server's IP address has not been recently blacklisted.

Solution 1Recommended

Contact Your Email Administrator or Cox.net Support

Investigate the rejection with your email provider or Cox.net to clear sender reputation flags and Cloudmark filtering.

Cloudmark is a spam and security filtering system utilized by Cox.net. When emails are returned as undeliverable, it usually means your sending domain or IP address is being blocked by their email rejection policies due to sender reputation.

Because this is a server-level block, it cannot be fixed by changing settings in your local applications, such as Microsoft Word or Outlook.

Review the bounce message

Open the undeliverable email return notification and locate the specific error code or link provided by Cloudmark.

Contact your IT department

If you are using a workplace email, forward the bounce message to your email administrator so they can verify your domain's authentication records (SPF, DKIM, DMARC).

Submit an unblock request

If you are the email server administrator, reach out to Cox.net Support or navigate to the Cloudmark remediation portal to request the removal of your IP address from their blocklist.

Unrelated to Microsoft Word: Although this issue was queried in a document forum, email routing and Cloudmark filtering are completely unrelated to Microsoft Word. It is strictly a mail server delivery issue.

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Frequently Asked Questions

Why does Cloudmark block emails to Cox.net?

Cloudmark uses automated filtering based on spam reports and sender reputation. If your IP address or domain lacks proper authentication or is suspected of sending bulk spam, Cloudmark will reject the messages to protect Cox.net users.

How can I improve my sender reputation to avoid rejected emails?

Ensure your domain has correctly configured SPF, DKIM, and DMARC records. Additionally, avoid sending unsolicited bulk emails and make sure your server is not an open relay.

Can Microsoft Word or my email app cause this bounce?

No. Email rejection by Cloudmark happens at the mail server level. Applications like Microsoft Word or local email clients do not trigger these specific undeliverable bounces.

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