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Fix Office 365 Email Rejected by Gmail (Error 550 5.7.350)
Question details
The user is attempting to send emails from a custom domain hosted on Microsoft 365 to Gmail recipients but faces delivery failures due to authentication issues.
- Product
- Microsoft 365 / Exchange Online
- Device & OS
- not provided
- Scenario
- Sending outbound email messages from a custom Microsoft 365 domain to Gmail accounts.
- Observed behavior
- Emails are rejected by Gmail and returned as suspected spam, generating SMTP error 550 5.7.350 because domain authentication (SPF, DKIM, DMARC) is incomplete or incorrect.
Ensure you have administrative access to both your Microsoft 365 Admin Center and your domain's DNS hosting provider (such as GoDaddy, Cloudflare, or Namecheap) to modify the necessary domain records.
Configure SPF, DKIM, and DMARC Records for Microsoft 365
Properly setting up these three email authentication protocols will verify your domain's identity to Gmail, removing the spam suspicion and resolving the 550 5.7.350 error.
Gmail has implemented strict sender guidelines that require all external domains to be fully authenticated. Error 550 5.7.350 means Gmail could not verify that your email legitimately came from your domain.
Log in to your DNS provider's dashboard. Create a new TXT record for your domain with the host set to '@' and the value 'v=spf1 include:spf.protection.outlook.com -all'. This authorizes Microsoft 365 to send emails on your behalf.
Navigate to the Microsoft 365 Defender portal. Go to 'Policies & Rules' > 'Threat Policies' > 'DomainKeys Identified Mail (DKIM)'. Select your custom domain and choose the option to create DKIM keys. The system will generate two CNAME records.
Take the two CNAME records generated by Microsoft 365 and add them to your DNS provider's settings. Once saved, return to the Defender portal and enable the toggle to 'Sign messages for this domain with DKIM signatures'.
In your DNS provider, create a new TXT record with the host '_dmarc' and the value 'v=DMARC1; p=quarantine; rua=mailto:your-email@yourdomain.com;'. This tells Gmail how to handle emails that fail SPF or DKIM checks.
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Frequently Asked Questions
Why does Gmail suddenly reject emails from my Office 365 domain?
Gmail frequently updates its spam protection policies and now strictly requires sending domains to use SPF, DKIM, and DMARC authentication. If your domain lacks these records or they are misconfigured, Gmail blocks the messages to protect users from spoofing and phishing.
Can I resolve Error 550 5.7.350 without accessing my DNS settings?
No, resolving this error requires modifying your domain's DNS zone files to add the necessary authentication records. If you do not have administrative access, you will need to contact your IT administrator or your domain hosting provider for assistance.
How can I verify that my SPF, DKIM, and DMARC records are working?
You can use free online DNS checker tools like MxToolbox or the Microsoft Remote Connectivity Analyzer. These tools allow you to input your domain and will quickly analyze if your SPF, DKIM, and DMARC records are correctly published and aligned.
