Summary
Apple's CS01 bounce code signifies a rejection due to local policy, primarily related to spam filtering. While Apple's documentation doesn't explicitly list CS01, the underlying RFC code indicates 'Delivery not authorized'. Experts and marketers agree that email forwarding significantly exacerbates this issue. Forwarding often breaks SPF, DKIM, and DMARC authentication because the forwarding server isn't authorized to send on behalf of the original sender's domain, causing messages to be flagged as spam. Some suggest the forwarder itself might be blocked due to forwarding spam. Implementing SRS, ensuring proper SPF/DKIM configuration, and contacting the email provider are recommended to mitigate these issues.
Key findings
- CS01 = Local Policy Rejection: CS01 indicates a rejection due to local policy, usually related to spam filtering on the recipient's server (Apple).
- Forwarding Breaks Authentication: Email forwarding disrupts SPF, DKIM, and DMARC authentication, leading to failed authentication checks.
- Forwarder Might Be Blocked: In some cases, the forwarder itself might be blocked due to a history of forwarding spam.
- RFC 5.7.1 Implication: Underlying RFC 5.7.1 code signifies 'Delivery not authorized,' requiring further interpretation by the receiving mail system.
Key considerations
- Implement SRS: Implementing SRS (Sender Rewriting Scheme) is crucial for maintaining deliverability during email forwarding.
- Proper SPF/DKIM Configuration: Ensure proper SPF and DKIM configuration to authorize forwarding servers to send emails on behalf of the original domain.
- Contact Email Provider: Contact the email provider for specific details about the CS01 bounce code and potential resolutions.
- Monitor Forwarder Reputation: Monitor the reputation of the email forwarder being used to ensure it's not contributing to deliverability issues.
What email marketers say
12 marketer opinions
Apple bounce code CS01 signifies a message rejection due to local policy, typically related to spam filtering. Email forwarding often exacerbates this issue by disrupting SPF, DKIM, and DMARC authentication, leading to the forwarded message being flagged as spam. This occurs because the forwarding server is often not authorized to send on behalf of the original sender's domain. Implementing SRS (Sender Rewriting Scheme) and ensuring proper SPF and DKIM configuration can mitigate these deliverability problems.
Key opinions
- CS01 Definition: CS01 error code means 'Message rejected due to local policy', which indicates a problem with the recipient's mail server spam filtering policies.
- Forwarding Impact: Email forwarding can break SPF, DKIM, and DMARC authentication because the forwarding server is not authorized to send on behalf of the original sender's domain.
- Authentication Failure: When authentication fails, receiving mail servers flag the forwarded email as potentially spam, leading to bounces.
- Mailer-Daemon: The bounce message (mailer-daemon) is often generated by the forwarder.
Key considerations
- Implement SRS: Implementing SRS (Sender Rewriting Scheme) is crucial to maintain deliverability when forwarding emails by rewriting the sender address.
- Proper Authentication Setup: Ensure proper SPF and DKIM configuration to allow forwarding servers to send emails on behalf of the original domain.
- Email Deliverability Issues: Forwarding emails without proper SPF and DKIM handling can lead to deliverability issues, including bounces.
- Domain Setup: Check and update the domain's DNS records to correctly authorise the forwarding server.
Marketer view
Email marketer from ReturnPath (now Validity) explains how forwarded emails often fail SPF checks because the original sender's SPF record does not authorize the forwarding server. SRS can mitigate this by rewriting the sender address.
14 Dec 2023 - ReturnPath
Marketer view
Email marketer from Postmark explains that forwarding emails without proper SPF and DKIM handling can lead to deliverability issues, including bounces. When an email is forwarded, the original SPF record might no longer be valid, leading to the email being flagged as spam.
26 Aug 2021 - Postmark
What the experts say
4 expert opinions
The Apple CS01 bounce code indicates that an email was rejected due to a local policy, most often spam filtering. Email forwarding worsens this issue because it can break SPF and DKIM authentication. This broken authentication then flags the emails as spam because forwarding servers are not often authorized by the original domain to send emails on their behalf. As a result, the expert advice is that you should use a proper email authentication setup to avoid being flagged by Apple's servers.
Key opinions
- CS01 is a spam block: The CS01 error code is a spam block from Apple's local policy rules.
- Forwarders Blocked: It's possible the forwarder itself is known to forward spam and is thus blocked.
- Authentication failures: Email forwarding without proper SPF/DKIM configuration can cause authentication failures, leading to emails being flagged as spam.
- Deliverability impact: Broken authentication due to email forwarding negatively impacts email deliverability.
Key considerations
- Proper Authentication Setup: Use a proper email authentication setup to avoid being flagged by Apple's servers.
- Verify forwarder reputation: Ensure the email forwarder being used has a good reputation and isn't known for forwarding spam.
- Review forwarding configuration: Review SPF and DKIM records to ensure they are properly configured to allow the forwarding server to send emails on behalf of the original domain.
Expert view
Expert from Email Geeks explains that it might just be that forwarder forwards lots of spam and is blocked.
18 Jan 2023 - Email Geeks
Expert view
Expert from Email Geeks explains that CS01 is specifically a spam block and suggests it's likely forwarding.
2 Apr 2025 - Email Geeks
What the documentation says
4 technical articles
Apple's bounce code CS01 is not explicitly defined in Apple's general support documentation, but the underlying RFC 5.7.1 code signifies 'Delivery not authorized, message refused,' indicating policy violations on the recipient's server. Email forwarding often breaks DMARC authentication, as forwarders are typically not authorized to send on behalf of the original domain, leading to email rejection. Microsoft's documentation corroborates that NDRs (Non-Delivery Reports) with 5.7.1 codes often indicate policy violations, which can be exacerbated by email forwarding complicating authentication.
Key findings
- CS01 Undocumented: Apple's official support documentation provides only general information and doesn't specifically address CS01.
- RFC 5.7.1: The underlying RFC 5.7.1 code indicates 'Delivery not authorized, message refused,' pointing to recipient server policies.
- Forwarding Breaks DMARC: Email forwarding commonly breaks DMARC authentication because forwarders usually aren't authorized to send on behalf of the original domain.
- Policy Violations: NDRs with 5.7.1 codes often mean policy violations, with forwarding complicating authentication.
Key considerations
- Contact Email Provider: Contact the email provider for specific details about the CS01 bounce code.
- Check Forwarding Configuration: Check and update your email forwarding configuration to ensure DMARC compliance.
- Understand Recipient Policies: Understand that receiving servers reject emails based on their own defined policies.
- Address DMARC Failures: Address DMARC failures resulting from forwarding to improve email deliverability.
Technical article
Documentation from Apple Support explains that they provide general information about bounce messages, but does not specifically list error code CS01. Suggests contacting the email provider for specific details.
7 Apr 2025 - Apple Support
Technical article
Documentation from Microsoft says that NDRs with codes like 5.7.1 often indicate policy violations. While they don't specifically mention CS01, the general explanation is that the recipient server's policies are rejecting the email. Forwarding can complicate authentication and trigger these policies.
21 Jun 2022 - Microsoft
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