The Error Message You Are Seeing

If you sent an email from your Rebel Cloud Email account to a Yahoo, Gmail, or Outlook address and received an automated bounce-back message similar to this:

Server returned an error: 550 5.7.9 This mail has been blocked because the sender is unauthenticated. Yahoo requires this sender to authenticate with DMARC (DMARC-aligned DKIM or DMARC-aligned SPF). Authentication results: DKIM = FAILURE - SPF yourdomain.com with ip ... = FAILURE.

Don't worry—this is a standard authentication block implemented by major email networks, and it can usually be resolved in a few minutes by updating a single record in your domain's DNS settings.

Why Did This Happen?

Major email providers like Yahoo and Google require all custom domain emails to prove they are legitimate senders to prevent spam and identity spoofing. They do this using three security protocols:

1. SPF (Sender Policy Framework): A public list in your domain's DNS stating which mail servers are allowed to send emails on your behalf.

2. DKIM (DomainKeys Identified Mail): A digital security signature attached to outgoing messages to prove they were not altered in transit.

3. DMARC (Domain-based Message Authentication): The security guard rule set by recipient networks that checks your SPF and DKIM.

The Cause of the Bounce:

When Yahoo or Gmail inspects your outgoing email, DMARC rules state that your email only needs EITHER a valid SPF record OR an aligned DKIM signature to pass.

If your domain's SPF record is missing, incorrectly formatted, or contains a typo (such as a missing space), recipient servers cannot verify that Rebel's Cloud Email platform is authorized to send mail for you. Both SPF and DKIM fail, and the message is blocked immediately.

How to Fix It (Step-by-Step)

To resolve this issue immediately, you need to ensure your domain has the correct SPF record published in its DNS management panel.

Step 1: Check & Update Your SPF Record

Log in to your Rebel Account (or your external DNS provider if your nameservers are hosted elsewhere) and add or update the following TXT Record:

CRITICAL SYNTAX NOTE: Make sure there is a single space between spf.cloudus.oxcs.net and ~all. If they are merged together (e.g., spf.cloudus.oxcs.net~all), the record will fail, and emails will continue to bounce!

Step 2: Ensure You Only Have ONE SPF Record

A domain can only have one SPF record. If you already have an existing SPF record for another service (like Shopify, Microsoft 365, or Mailchimp), merge them into a single record rather than creating a second TXT record:

• Incorrect (Multiple Records):

  Record 1: v=spf1 include:spf.cloudus.oxcs.net ~all

  Record 2: v=spf1 include:shops.shopify.com ~all

• Correct (Merged Record):

  v=spf1 include:spf.cloudus.oxcs.net include:shops.shopify.com ~all

Frequently Asked Questions

Do I need to manually configure DKIM records?

No. The Rebel Cloud Email platform automatically stamps outgoing mail with a backend security signature (oxcloud.zone). Because DMARC rules only require either SPF or DKIM to align, having the correct include:spf.cloudus.oxcs.net SPF record is 100% sufficient to pass Yahoo and Gmail's strict security checks.

What should my MX records look like for Rebel Cloud Mail?

To make sure you can receive incoming mail cleanly, ensure your domain has these four MX records set:

• mx001.rebel.xion.oxcs.net

• mx002.rebel.xion.oxcs.net

• mx003.rebel.xion.oxcs.net

• mx004.rebel.xion.oxcs.net

How long until my emails start delivering?

DNS changes typically take 15 to 30 minutes to propagate across the global internet (though in rare cases, it can take up to 4 to 24 hours). Once updated, send a test message to a Yahoo or Gmail address to confirm delivery!

Still Having Issues?

If you have updated your SPF record and are still receiving bounce notifications:

1. Please download the .EML file (raw email sample) of the bounced message using our Rebel Cloud Email Webmail guide.

2. Open a support ticket with Rebel attaching the .EML file so our team can inspect the exact headers and resolve any remaining delivery hurdles for you!