Forwarding Email from Gmail to Outlook When SRS Is Not Supported

If you’ve tried forwarding email from Gmail to Outlook, especially Microsoft 365 Outlook accounts based in China, and hit authentication errors like SPF or DKIM failures, you’re not alone. Microsoft’s spam filters have become stricter, and forwarding without proper authentication breaks delivery.

The root issue? Gmail doesn’t rewrite the sender’s envelope when forwarding. And without Sender Rewriting Scheme (SRS), messages sent through IPv6 that fail SPF or DKIM will often be blocked by Outlook. Google does support SRS, but only for certain Workspace tiers, and if it’s not available in your Admin Console, you're out of luck.

Here’s how we solved it with a reliable, scalable workaround using Mailgun, and how we got there faster with the help of AI.

The Problem

We wanted to forward all emails sent to a Gmail inbox (hosted on a custom domain) to a Microsoft 365 Outlook inbox used by our team based in China.

Simple forwarding through Gmail worked for years, until Microsoft started rejecting messages with this error:

450 4.7.26 Service does not accept messages sent over IPv6 unless they pass either SPF or DKIM validation (body hash failure).

Since Gmail forwards mail “as-is,” the forwarded message appears to come from the original sender, not from Gmail. But because Gmail isn’t authorized to send on behalf of the original sender’s domain (and doesn't rewrite the envelope), Outlook blocks it. Without SRS, there's no fix from Gmail's side.

The Workaround: Use Mailgun to Handle Forwarding

Instead of relying on Gmail to forward directly to Outlook, we used Mailgun to act as a smart relay:

1. Set up a subdomain (e.g. forward.yourdomain.com) and point its DNS to Mailgun.
2. Configure Mailgun’s DNS records: SPF, DKIM, and MX.
3. Create a Mailgun route to forward incoming messages to the Outlook address.
4. Set Gmail to forward to your Mailgun subdomain.

Mailgun rewrites the envelope and handles forwarding in a way that preserves SPF/DKIM integrity, allowing Outlook to accept the message.

Why This Works

Mailgun is a dedicated email service that can handle incoming messages, rewrite the envelope sender, and forward the email with all necessary authentication intact. This bypasses Gmail’s limitations and makes the messages fully compliant with Microsoft 365’s spam protection requirements.

What About Replies?

This solution only handles inbound email forwarding. When your Outlook user replies (e.g. susan@yourdomain.onmicrosoft.com), the message is sent directly from their Microsoft 365 mailbox. There’s no routing involved — so replies are fast, clean, and authenticated.

A Little Help From AI

As a bonus, we were able to troubleshoot, test, and complete this setup much faster thanks to AI tools. From decoding cryptic bounce errors to streamlining DNS configuration and routing logic, AI helped reduce the trial-and-error phase and speed up our implementation.

Conclusion

If you're trying to forward Gmail to Outlook in a professional or cross-border context (like users in China), and Gmail's native SRS support isn't available, this Mailgun relay method is a smart fix. It's reliable, scalable, and avoids the messy deliverability issues that come with broken SPF/DKIM chains.

Need help setting this up? Drop us a message — we’ve done this for ourselves (with a little help from AI) and can help you do it too. Neat is integrating AI solutions to streamline all aspects of marketing, check out our AI Integration page for more information.