Office 365 / Microsoft 365 SMTP Error Codes
Master email delivery to Office 365 / Microsoft 365's Exchange Online. Understand regional routing, tenant policies, and Microsoft's spam filtering.
💼 About Office 365 / Microsoft 365 Email
Office 365 is a subscription-based suite of productivity software and services developed by Microsoft. This guide focuses on their email hosting solution: Exchange Online.
Platform Overview
- • Launched June 28, 2011 as Office 365
- • Rebranded to Microsoft 365 in April 2020
- • Dominant enterprise email platform
- • Exchange Online backbone
Technical Architecture
- • Regional datacenter model
- • Proprietary spam filters (Forefront)
- • Uses Spamhaus and other RBLs
- • No feedback loop available
🌐 MX Server Patterns
Office 365 uses different MX patterns based on tenant configuration:
*.mail.protection.outlook.com
Standard Exchange Online Protection
*.mail.eo.outlook.com
Exchange Online direct routing
*.prod.protection.outlook.com
Production protection services
⚠️ Each MX returns 2 IPs from regional clusters (EUR04, NAM03, JPN01, etc.). You cannot use wildcards - must route by cluster or destination IP.
📊 Proprietary Header Analysis
Use Microsoft's Message Header Analyzer to check these critical scores. Office 365 has additional filtering rules compared to Outlook/Hotmail consumer mailboxes.
SCL (Spam Confidence Level)
0-9Message spam score. 5-6 = suspected spam, 7+ = high confidence spam
Default threshold: 5
PCL (Phishing Confidence Level)
1-8Likelihood the message is phishing
Any positive value indicates phishing risk
BCL (Bulk Complaint Level)
1-9Bulk sender complaint rate. Higher = more complaints
Default threshold: 7
Office 365 SMTP Error Codes Explained
421 4.7.0
Temporary FailureExchange Online is temporarily unavailable. Retry the delivery later.
421 4.7.0 usually reflects a transient Microsoft service interruption. Respect the retry interval or Microsoft may extend the deferral.
How to Fix:
- ✓Implement exponential backoff when retrying the message.
- ✓Monitor the Microsoft 365 Service Health dashboard for incidents.
- ✓Spread retries across multiple connectors to avoid overwhelming a single region.
- ✓Document the timeline in case recipients open a support ticket.
451 4.4.4 (ATTR5)
Temporary Server ErrorExchange Online experienced a transient error processing the message.
ATTR5 indicates the Microsoft receiving server encountered an internal failure. The message can be retried shortly.
How to Fix:
- ✓Retry with exponential backoff rather than immediate reconnects.
- ✓Track the frequency of ATTR5 events to spot regional instability.
- ✓Escalate to Microsoft support if the error repeats for the same tenant.
- ✓Keep backup MX routes ready if the tenant enforces hybrid delivery.
451 4.7.500 (S77719)
Server BusyThe Microsoft receiving server is overloaded and cannot accept more connections.
Microsoft throttles inbound traffic when a datacenter is under load. The block usually clears within minutes if you slow down.
How to Fix:
- ✓Implement adaptive throttling for Outlook domains when this error appears.
- ✓Distribute mail across time zones rather than one global burst.
- ✓Monitor connection and message concurrency per tenant.
- ✓Alert operations teams when S77719 rates spike beyond normal baselines.
452 4.5.3 (ATTR38)
Region MismatchThe recipient list spans multiple Microsoft regions and cannot be processed in one transaction.
Multi-tenant organisations often host users across regions. Microsoft requires you to send separate transactions per region.
How to Fix:
- ✓Segment distribution lists so each message targets one Microsoft region.
- ✓Review hybrid routing or smart hosts that override EOP’s decisions.
- ✓Ask the recipient organisation to consolidate MX routing where possible.
- ✓Leverage per-region connectors in Microsoft 365 if you manage the tenant.
451 4.4.62 (ATTR35)
Wrong RegionThe message was routed to the wrong Microsoft 365 region.
Misconfigured DNS or routing rules can send mail to an incorrect datacenter. Microsoft rejects the message to prevent loops.
How to Fix:
- ✓Verify the recipient’s MX records resolve to the intended geography.
- ✓Check custom connectors or transport rules that might reroute mail.
- ✓Coordinate with the recipient admin to confirm regional configuration.
- ✓Update your smart host list to include regional endpoints.
451 4.7.650
ReputationExchange Online Protection temporarily blocked the sender pending review.
4.7.650 indicates Microsoft is throttling the sender for possible spam or compromised accounts. Continued abuse escalates to 5xx blocks.
How to Fix:
- ✓Check Microsoft 365 Enhanced Filtering or Security Center for alerts.
- ✓Ensure no compromised accounts or applications are sending mail.
- ✓Reduce send volumes and improve list hygiene for Outlook recipients.
- ✓Prepare a remediation summary before contacting Microsoft support.
550 5.4.1
Access DeniedThe recipient mailbox is invalid or blocked at the tenant level.
If the address is wrong or an admin has blocked inbound mail, Microsoft returns 5.4.1. Treat invalid addresses as hard bounces.
How to Fix:
- ✓Verify address spelling and remove invalid recipients from future sends.
- ✓Ask the recipient to confirm their mailbox or alias configuration.
- ✓Request a tenant-level allow entry if the recipient intentionally blocks external mail.
- ✓Update CRM or marketing databases to prevent repeat attempts.
554 5.4.14 (ATTR34)
Mail LoopMicrosoft detected a mail routing loop and stopped the delivery.
Misconfigured forwards or hybrid routing can create loops. Microsoft drops the message when the hop count exceeds its safety threshold.
How to Fix:
- ✓Inspect forwarding rules for both sender and recipient tenants.
- ✓Verify connectors and smart hosts do not route mail back to Exchange Online.
- ✓Check third-party security appliances that might re-inject mail incorrectly.
- ✓Work with recipient admins to resolve policy conflicts causing the loop.
550 5.7.511
Sender BlockExchange Online Protection banned the sending IP or domain for abuse.
5.7.511 is Microsoft’s strongest block for IP reputation problems. You must show remediation before Microsoft removes the ban.
How to Fix:
- ✓Forward the original NDR to delist@messaging.microsoft.com with full headers.
- ✓Document corrective actions (list cleaning, compromised account remediation, etc.).
- ✓Pause traffic until Microsoft confirms the unblock to avoid extended bans.
- ✓Warm a fallback IP in case the review process takes several days.
550 5.7.606
IP BlockMicrosoft blocked the sending IP address via Exchange Online Protection.
5.7.606 often includes an AS number. You must submit a mitigation request at sender.office.com after resolving the abuse.
How to Fix:
- ✓Visit https://sender.office.com/ and submit a mitigation ticket with evidence.
- ✓Fix the behaviour (malware, open relay, spam) before asking for delisting.
- ✓Rotate traffic to clean IPs while the review is in progress.
- ✓Set up ongoing monitoring so issues are caught before Microsoft blocks again.
550 5.7.708
IP ReputationMicrosoft blocked the IP because the traffic resembles high-volume spam or suspicious automated mail.
5.7.708 is common for residential or cloud IP ranges without a sending history. Microsoft requires proof that the mail is legitimate bulk traffic.
How to Fix:
- ✓Move production mail to a dedicated, properly warmed IP range.
- ✓Provide Microsoft with logs showing opt-in proof and engagement when requesting mitigation.
- ✓Avoid sending from dynamic or consumer broadband space.
- ✓Implement rate controls and authentication that prove you own the domain.
550 5.7.350
Tenant PolicyThe recipient tenant rejected the message due to custom policies or allow/block lists.
Tenants can configure stricter requirements than Microsoft’s defaults. If your domain fails their custom rules, you will see 5.7.350.
How to Fix:
- ✓Ask the recipient administrator to add your domain or IP to their allow list.
- ✓Share authentication and encryption details to prove trustworthiness.
- ✓Split transactional and marketing streams to meet different policy expectations.
- ✓If multiple tenants reject you, evaluate your baseline list hygiene and cadence.
550 5.7.64
Tenant PolicyThe recipient tenant has blocked your domain or IP because of poor reputation or user reports.
TenantLevelReputation is controlled by each organisation. Even if Microsoft accepts the mail, the tenant can still refuse it.
How to Fix:
- ✓Engage directly with the tenant to understand the reason for the block.
- ✓Provide evidence of opt-in and remediation to regain trust.
- ✓Improve targeting so that only engaged users at that tenant receive mail.
- ✓Request an allow entry once compliance improvements are in place.
Office 365 Best Practices
Regional Considerations
- •Office 365 uses regional datacenters
- •Recipients must be in same region per transaction
- •MX records vary by region (EUR, NAM, JPN, etc.)
- •Cannot use wildcard routing
Authentication & Headers
- •SPF, DKIM, DMARC required
- •Monitor SCL, PCL, BCL scores
- •Use Message Header Analyzer
- •Check proprietary headers for issues
Tenant Settings
- •Each tenant has custom settings
- •Admin can override global policies
- •Whitelisting may be required
- •Work with recipient admins
Monitoring
- •No feedback loop available
- •Check service health dashboard
- •Monitor bounce patterns
- •Track regional performance
🏢 Tenant-Specific Considerations
Admin Controls
- • Each tenant has custom spam thresholds
- • Admins can override global policies
- • Transport rules may block specific senders
- • Safe/blocked sender lists per tenant
- • Default BCL threshold is 7 (but customizable)
Resolution Strategy
- • Work directly with recipient admins
- • Provide full message headers
- • Check tenant-specific allowlists
- • Review custom transport rules
Technical Configuration
Office 365 Routing
EUR04: Europe (Dublin, Amsterdam)
GBR01: Great Britain
NAM03: North America
JPN01: Japan
AUS01: Australia
1. Destination IP clustering
2. Regional datacenter mapping
3. Per-tenant MX resolution
- Forefront (proprietary filter)
- Microsoft Antispam (proprietary)
- Third-party RBLs (e.g., Spamhaus)
- Additional rules vs consumer Outlook/Hotmail
🚨 Delisting Process
For 550 5.7.511 Errors
- Forward complete bounce message to delist@messaging.microsoft.com
- Include sending IP address and domain
- Explain remediation steps taken
- Wait 24-48 hours for response
For 550 5.7.606 Errors
- Visit https://sender.office.com/
- Complete the delisting form
- Provide detailed information about your sending practices
- Implement all recommended changes
- Monitor status through the portal
⚠️ Important: If email is being marked as spam or blocked, it's generally due to the recipient domain's O365 administrator settings. You'll need to collaborate with the recipient and provide them with the headers of affected messages for whitelisting.
Office 365 Resources
Need Help with Office 365 Delivery?
Office 365's regional architecture and tenant-specific settings can be complex to navigate. Let us help you optimize delivery and resolve tenant-level blocks.