When sending emails to Outlook or Microsoft 365 recipients, you may encounter 550 5.7.x error codes that block message delivery. These errors are triggered by issues related to domain reputation, IP blacklisting, tenant misconfiguration, or sending behavior.
Each error code (e.g., 550 5.7.501, 550 5.7.708) provides specific insights into what went wrong—such as banned domains, unauthorized relays, or exceeded sending thresholds. Resolving these errors typically involves:
Ensuring SPF, DKIM, and DMARC records are properly set up
Monitoring domain and IP reputation
Verifying correct Microsoft 365 tenant configurations
Managing sending volumes and warming up domains gradually
By identifying the exact error code and applying the appropriate fix, you can improve your email deliverability and maintain a strong sender reputation with Outlook-based recipients.
Below is a guide to understand the cause and apply the right fix.
Cause: Outlook has flagged your sending domain as banned or blacklisted.
Fix:
Check if the domain is listed on common blocklists (MXToolbox, Spamhaus).
Rotate to a new, clean sending domain.
Improve domain reputation with warm-up and authenticated sending (SPF, DKIM, DMARC).
Open a support request with Microsoft if the ban persists.
Cause: The domain is not correctly associated with the Microsoft 365 tenant.
Fix:
Verify that the domain is added and validated in your Outlook/Microsoft 365 tenant.
Ensure MX and TXT records are correctly set up.
If using a third-party SMTP, confirm the domain is properly authorized for relaying.
Cause: Outlook blocks messages from tenants with low reputation or trial licenses.
Fix:
Contact Microsoft support to request an exception for your tenant.
If on a trial tenant, upgrade to a licensed Microsoft 365 account.
Send at lower volume while warming up the domain.
Cause: Your Microsoft tenant has crossed Outlook’s daily/volume limits.
Fix:
Reduce sending volume (10–15/day per mailbox for new accounts).
Stagger campaigns across multiple verified domains and mailboxes.
Monitor tenant activity in the Exchange admin center for limits and throttling.
Cause: Outlook banned the sending IP, often due to spam complaints or poor reputation.
Fix:
If using a shared SMTP, contact your provider for IP rotation.
Move to a dedicated IP/server (SmartServer or equivalent).
Pair the domain with a clean, non-blacklisted IP.
Cause: Outlook identified bulk/spam-like behavior from your domain.
Fix:
Slow down sending volume and avoid sudden spikes.
Improve personalization and engagement (avoid spammy content).
Monitor open/reply rates — maintain positive sender reputation.
Warm up the domain before scaling campaigns.
Cause: Domain authentication failed (SPF, DKIM, DMARC) or unauthorized relay attempt.
Fix:
Ensure SPF, DKIM, and DMARC are correctly configured for the sending domain.
Verify the “MAIL FROM” domain matches your authenticated records.
If relaying via Outlook, confirm the domain is authorized in the tenant.
Still Need Help?If you're still facing issues after following the fixes above, feel free to contact our support team. We’ll help diagnose your sending setup and guide you toward full deliverability.
