Fix Gmail Rejecting Microsoft 365 Emails with Error 550 5.4.300
Question details
Microsoft 365 emails sent to Gmail users are bouncing back with an unsolicited-mail rate limit error.
- Product
- Microsoft 365
- Device & OS
- not provided
- Scenario
- Sending distribution-list messages or standard emails from a Microsoft 365 account to Gmail addresses.
- Observed behavior
- Emails are rejected by Gmail, returning an error code 550 5.4.300, which typically indicates missing email authentication records or unusual sending volume.
Ensure you have global administrator access to your Microsoft 365 admin center and login credentials for your custom domain's DNS hosting provider.
Enable and Configure DKIM, SPF, and DMARC Records
Proper email authentication verifies your domain's identity, preventing strict providers like Gmail from flagging your Microsoft 365 emails as spam.
Gmail strictly enforces email authentication to protect users from phishing. If your Microsoft 365 domain lacks proper SPF, DKIM, or DMARC records, Gmail is likely to reject your messages with error 550 5.4.300.
Log in to the Microsoft 365 Defender portal. Navigate to Policies & Rules > Threat Policies > Email Authentication Settings > DKIM.
Select your custom domain and toggle on DKIM signing. Follow the prompt to generate CNAME records and publish them in your DNS provider's dashboard.
Access your DNS provider settings and ensure your domain has a valid SPF TXT record pointing to Microsoft 365 (e.g., v=spf1 include:spf.protection.outlook.com -all).
Create a DMARC TXT record in your DNS settings to instruct email providers on how to handle unauthenticated emails from your domain.
Audit Sending Activity and Spam-Like Behavior
A compromised account or a misconfigured distribution list sending bulk spam can damage your domain's IP reputation.
Contact Microsoft 365 Support
If you have correctly configured your DNS records but Gmail continues to reject messages, Microsoft Support can investigate potential IP blocklist issues.
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Frequently Asked Questions
What does error 550 5.4.300 mean?
The 550 5.4.300 error indicates an unsolicited-mail rate limit issue. It usually means the receiving server (like Gmail) rejected the email because your domain lacks proper authentication (SPF, DKIM, DMARC) or has a poor sender reputation.
Why did Gmail suddenly start rejecting my Microsoft 365 emails?
Google regularly updates its email security policies. Recently, they began strictly enforcing SPF, DKIM, and DMARC authentication for bulk senders and custom domains. If your domain isn't fully authenticated, Gmail will reject the messages to prevent spam.
Do I need DMARC if I already have SPF and DKIM configured?
Yes, having a DMARC policy is highly recommended and sometimes required by major providers like Gmail and Yahoo. DMARC ties SPF and DKIM together, giving you control over how unauthenticated emails pretending to be from your domain are handled.
