What is the primary cause of the '5.7.1 Email rejected due to transformation error' from Apple?

The 5.7.1 Email rejected due to transformation error from privaterelay.appleid.com primarily indicates that your sending domain is not registered or properly authenticated with Apple's Private Email Relay service. This is required for emails to Sign in with Apple addresses.

How do I register my sending domain with Apple for Private Relay?

You must register all your sending domains (the domains in your From address) with your Apple Developer account. This authenticates your domain and permits Apple to relay your emails.

Is there a difference between 'Sign in with Apple' and 'Hide My Email' addresses regarding this error?

Yes, there is a distinction. Sign in with Apple uses @privaterelay.appleid.com addresses, which are affected by the 5.7.1 error and require domain registration. Hide My Email uses @icloud.com addresses, which typically have different deliverability requirements.

What if my domain is registered, but I'm still getting 'transformation error' bounces?

If your domain is already registered and verified, but you still experience these bounces, it could be a temporary service issue on Apple's end. It's also possible the private relay address is no longer valid. Monitor your bounce rates for consistency and consider contacting Apple's Postmaster team for further investigation.

What causes '5.7.1 Email rejected due to transformation error' bounces from Apple's privaterelay.appleid.com and how to fix it? - Troubleshooting - Email deliverability - Knowledge base

Encountering a '5.7.1 Email rejected due to transformation error' bounce message from privaterelay.appleid.com can be incredibly frustrating. This specific bounce often indicates a fundamental issue with how your email system interacts with

Apple's Private Relay service, which is a key component of their Sign in with Apple feature. It is a critical deliverability problem, especially for transactional emails like double opt-in messages.

When users opt to Hide My Email or use Sign in with Apple,

Apple generates a unique, anonymous email address (e.g., xxxx@privaterelay.appleid.com) to protect their privacy. Your email must be sent to this relayed address, and Apple's system then transforms it to the user's actual email. When this transformation fails, you receive the 5.7.1 Email rejected due to transformation error bounce.

Understanding the error

The 5.7.1 Email rejected due to transformation error bounce typically means that the email relay service, which converts the anonymous Apple-generated address to the user's real address, encountered an issue. This error is specific to

Apple's ecosystem, particularly when dealing with Sign in with Apple addresses. Unlike general bounces (or blocklist issues), this is often about a specific misconfiguration or a temporary service issue on Apple's side, rather than your sender reputation or content.

A common cause of this transformation error is that the sending domain for your emails has not been properly registered with

Apple's Private Email Relay service. If your domain isn't registered and authenticated, Apple cannot reliably relay your emails to the user's actual inbox, leading to a bounce. This is a security measure to prevent unauthorized senders from spamming private relay addresses. You can find more information about this process in Apple's developer documentation.

Typical bounce messageplaintext

5.7.1 Email rejected due to transformation error

Another possibility, though less common, is that the anonymous private relay email address you are sending to is no longer valid, or there was a temporary system issue on

Apple's end. This can sometimes happen during service updates or unexpected outages, causing widespread but temporary bounces. However, in most persistent cases, the issue lies with the sender's configuration.

Primary causes and troubleshooting

The distinction between Sign in with Apple and Hide My Email is important here. While both create anonymous email addresses, Sign in with Apple uses @privaterelay.appleid.com addresses, whereas Hide My Email generates @icloud.com addresses. The 5.7.1 error is specifically tied to the former. For issues with general Apple domain bounces, other troubleshooting steps might apply. You can also review how Apple Private Relay affects soft bounces for more context.

Your organization must adhere to

Apple's program rules for Sign in with Apple, which mandates that all sending domains are registered and authenticated. Without this, even if the user's private relay address is active, the email cannot be transformed and delivered. This is a crucial step for preventing policy-related bounce messages from Apple.

Sign in with Apple (privaterelay.appleid.com)

Purpose: Allows users to sign into third-party apps and websites using their Apple ID, providing a privacy-focused alternative.
Email Relay: Generates a unique, anonymous email address (e.g., xxxx@privaterelay.appleid.com) that forwards to the user's real email address.
Sender Requirement: Requires your sending domains to be explicitly registered and verified with your Apple Developer account.

Before registering sending domains

High bounce rates: Experiencing 5.7.1 Email rejected due to transformation error for private relay addresses.
Delivery failures: Critical transactional emails like double opt-in messages not reaching users.
Sender reputation risk: Accumulated bounces can negatively impact overall sender reputation and potentially lead to being placed on a blocklist or blacklist.

After registering sending domains

Improved deliverability: Emails to private relay addresses are successfully transformed and delivered.
Enhanced user experience: Seamless user onboarding and communication flow, especially for Apple users.
Reduced bounce rates: Maintains a healthier sender reputation and avoids unnecessary blocklisting (or blacklisting) issues.

In some instances, the issue may not be directly related to your configuration but rather to a temporary glitch on

Apple's end. While

Apple is generally quite reliable, service disruptions can occur. If you notice a sudden, widespread increase in these bounces that affects many of your private relay recipients simultaneously, and your domain is already correctly registered, it might indicate a system-wide problem that

Apple is likely already working to resolve.

Always ensure that the user ID corresponding to the private relay address is still active and valid within your system. If a user has deactivated their account or opted out of communications through

Apple's privacy settings, the private relay address may no longer transform to a valid destination, resulting in this specific bounce error. Regular list hygiene is important for maintaining good email deliverability across all recipient types.

Solutions and best practices

The most critical step to fix the 5.7.1 Email rejected due to transformation error is to ensure that all domains you use to send emails are properly registered and authenticated with your

Apple Developer account. This is a mandatory requirement for interacting with Sign in with Apple's Private Email Relay. Without this registration,

Apple will reject your emails, as they are not coming from an authorized source. You can find more details on this specific configuration at Resend's knowledge base.

The registration process for your sending domains is typically handled within your

Apple Developer account. You will need to add and verify each domain that your application uses to send emails to

Apple Private Relay addresses. This involves proving ownership of the domain, often through DNS records like TXT records for SPF, DKIM, and DMARC. For more on email authentication protocols, consult our comprehensive guide.

After ensuring your domains are registered, actively monitor your bounce logs for 5.7.1 errors. If the issue persists, even with proper domain registration, consider reaching out to

Apple's Postmaster team or support, especially if you suspect a temporary issue on their side. You can often find contact information or troubleshooting guides at postmaster.icloud.com. Maintaining open communication channels can help resolve these specific deliverability challenges more quickly.

Additionally, ensure that the sender address (the From address) used for emails to private relay addresses is consistent with the one registered in your

Apple Developer account. Any mismatch can also trigger transformation errors, as

Apple enforces strict sender authentication for privacy reasons. It's a layer of security that complements standard email authentication protocols.

Cause	Solution
Unregistered sending domain: Your domain sending emails to @privaterelay.appleid.com is not registered with Apple.	Register domains: Add and verify all sending domains within your Apple Developer account.
Invalid private relay address: The user's private relay address is no longer active.	Clean lists: Remove bounced addresses from your mailing lists promptly.
Temporary Apple system issue: A rare, short-term problem on Apple's end.	Monitor and contact support: Track bounce rates and contact Apple Postmaster if bounces persist despite correct configuration.

Ensuring your email authentication records (SPF, DKIM, DMARC) are correctly set up is always a fundamental step for email deliverability. While these records primarily combat spam and phishing, their proper configuration contributes to your overall sending trustworthiness, which indirectly supports

Apple's stringent requirements. Regularly review your DMARC monitoring reports to identify any authentication failures that might contribute to deliverability issues.

Views from the trenches

Best practices

Always register and verify all domains used to send emails through your Apple Developer account.

Regularly check your bounce logs for 5.7.1 transformation errors specific to Apple Private Relay.

Keep your mailing lists clean by promptly removing invalid or inactive private relay addresses.

Maintain open communication with Apple's Postmaster team for persistent or widespread issues.

Common pitfalls

Forgetting to register a new sending domain with Apple's Private Email Relay service.

Assuming the issue is generic spam filtering or a blocklist problem without checking Apple's specific requirements.

Ignoring persistent 5.7.1 bounces, which can lead to larger deliverability issues.

Not removing private relay addresses that have become inactive or invalid from your lists.

Expert tips

If you're using Sign in with Apple, ensure you have a developer account to manage sending domains. It's often free.

The 'transformation error' typically means Apple couldn't convert the private ID to a valid address, often due to an unregistered sender.

Distinguish between @privaterelay.appleid.com (Sign in with Apple) and @icloud.com (Hide My Email), as they have different requirements.

When troubleshooting, check if the bounces started suddenly for multiple customers, which could indicate an Apple-side issue.

Expert view

Expert from Email Geeks says that the sending domain needs to be registered with Apple before you can send to them, as they encountered a similar problem with a loyalty card provider recently.

2022-07-20 - Email Geeks

Expert view

Expert from Email Geeks suggests confirming compliance with Apple's program rules and verifying that emails are sent from the registered email address with Apple for those addresses, as this could be the source of the issue.

2022-07-20 - Email Geeks

Summary

The 5.7.1 Email rejected due to transformation error from privaterelay.appleid.com is a clear indicator that your sending setup needs to align with

Apple's privacy and security protocols for Sign in with Apple addresses. The primary solution revolves around registering and authenticating your sending domains within your

Apple Developer account. Without this fundamental step,

Apple cannot process the email relay, leading to bounces.

While temporary system glitches on

Apple's end or invalid user IDs can occasionally contribute to these errors, the vast majority of cases point back to sender configuration. Prioritizing domain registration ensures that your email infrastructure is recognized and trusted by

Apple's stringent email relay service.

By proactively addressing these configuration requirements and maintaining good list hygiene, you can significantly reduce 5.7.1 transformation errors and ensure your important emails consistently reach users who have opted for Sign in with Apple

. This approach not only improves deliverability but also strengthens your overall email program and sender reputation.