Hotmail is blocking emails we are sending via gmail to members of our organization who use hotmail

Obtain the complete bounce message from Gmail and use its SMTP status code and diagnostic text to identify whether Hotmail rejected your domain for authentication, reputation, or another policy reason.^1^

1. Capture the full rejection details

In the Gmail account that sent the message:

1. Open the delivery failure notification.
2. Expand the technical details and retain:
   • The complete SMTP code, such as 550 5.7.515.
   • The text after the code.
   • The rejected recipient domain.
   • Any sending IP, host name, or Microsoft troubleshooting URL shown.
3. Redact email addresses and other personal information before sharing the report publicly.

The word “blocked” alone is not enough to select a fix. For example, references to Spamhaus or another blocklist indicate a spam/reputation block, while 550 5.7.515 specifically indicates insufficient authentication for the domain in the message’s From address.^2^

2. If the code is 550 5.7.515

This code means the custom domain shown in the From address did not meet Microsoft’s required authentication level. Microsoft applies the documented requirement to domains sending at least 5,000 messages per day to its consumer services, including Hotmail, Outlook.com, MSN, and Live.com.^3^

Have the administrator who controls your domain’s DNS and Google mail configuration:

1. Verify SPF passes. The SPF record must authorize every service that sends mail for your domain.
2. Enable DKIM signing in your Gmail/Google Workspace configuration. Confirm outgoing messages are signed using your organization’s domain, not an unrelated service domain.
3. Publish a DMARC TXT record. A valid policy can initially use p=none while authentication is being monitored.
4. Check DMARC alignment. At least one passing mechanism—SPF or DKIM—must align with the domain displayed in the From address.
5. Check every sending service. Mailing platforms, website forms, CRM systems, and forwarding gateways must also be authorized and aligned if they send using your domain.
6. Send a new test and inspect its authentication results for spf=pass, dkim=pass, and dmarc=pass.

Do not add a second independent SPF TXT record. Update the existing record to include every authorized sender; multiple SPF records can invalidate SPF evaluation. The exact DNS values must come from Google or any other service that actually sends for your domain rather than from a generic example.

3. If the NDR reports spam, reputation, or blocklist filtering

Microsoft considers the sending IP and domain, authentication, recipient-list quality, complaint rate, content, and sending patterns. It also may reject senders that fail reverse-DNS checks.^4^

Apply these measures:

• Avoid sending an organization-wide message as one large BCC distribution.
• Reduce sudden bursts and send only to members who expect the messages.
• Remove invalid, inactive, or repeatedly bouncing recipients.
• Ensure the From display name clearly identifies the nonprofit.
• Test a plain-text message without links, images, attachments, or a signature. If it succeeds, add those elements back individually.
• Check whether the NDR names a specific public blocklist. If so, follow that blocklist’s remediation process.
• If you use only Google Workspace to deliver mail, provide the rejection to Google Workspace support as well, because Google controls the outbound infrastructure.

Microsoft’s Sender Support page also provides sender reputation services and an Outlook.com deliverability issue-submission form. That form applies specifically to @hotmail.com, @outlook.com, @msn.com, and @live.com delivery problems. If authentication and sending practices are correct but rejection continues, submit the full NDR and representative timestamps through the Outlook.com deliverability troubleshooting form.^4^

4. Recipient-side action is only a limited workaround

Individual members can add your address or domain to Outlook.com → Settings → Mail → Junk email → Safe senders and domains. However, this is useful mainly when Microsoft accepts the message and then places it in Junk. If Gmail receives an SMTP rejection, the message never reaches the mailbox, so correcting sender authentication or reputation is the primary fix.

References

1. Microsoft’s Exchange & @hotmail.com / @outlook.com domains Spam problem. - Microsoft Q&A
2. Sent email in Outlook.com comes back "delivery failed" | Microsoft Support
3. Fix NDR error "550 5.7.515" in Outlook.com | Microsoft Support
4. Sender Support in Outlook.com | Microsoft Support