Overview
A business in Wandsworth contacted our team after multiple Microsoft 365 emails to partners and suppliers failed with non-delivery reports (NDRs). The failures created operational delays and raised concern about Office 365 misconfiguration, email security settings, and domain reputation.
As local IT support specialists, we carried out a full Microsoft 365 mail flow and deliverability investigation to find the root cause quickly and avoid unnecessary sender-side changes.
The Problem
The client reported that emails to several organisations were being rejected. Key warning signs were:
- Messages had previously delivered successfully
- Multiple recipient domains were affected
- Bounce messages referenced TLS and certificate errors
- Concerns were raised around SPF, DKIM, and DMARC
- The client feared blacklist or compromise issues
Because email is mission-critical, urgent diagnostics were required.
Our Diagnostic Approach
We used a structured troubleshooting process:
- SMTP and NDR analysis
- Microsoft 365 message trace and mail flow checks
- TLS handshake testing
- SPF, DKIM, and DMARC validation
- Secure email gateway header review (including Proofpoint and Barracuda routes)
- Outbound connector and routing verification
- Recipient certificate inspection
This confirmed exactly where delivery failed between sender and recipient infrastructure.
Key Findings
Our investigation confirmed the sender environment in Wandsworth was correctly configured. Authentication, outbound transport, and sender reputation were all healthy.
The delivery failures were external and fell into three categories.
1. Expired TLS Certificate
One recipient mail server presented an expired SMTP TLS certificate. Microsoft 365 rejected the connection because secure transport could not be established.
2. Invalid Certificate Chain
Another domain presented an incomplete or untrusted certificate chain during TLS negotiation. Delivery attempts were terminated by policy enforcement.
3. Restricted Microsoft 365 Partner Connector
A third organisation had a restrictive inbound partner connector that accepted only pre-approved source IPs or trusted certificate paths. Because the client’s messages were routed via a secure gateway, source validation failed and mail was rejected.
What We Verified on the Sender Side
To eliminate sender-side causes, we validated:
- Microsoft 365 outbound mail flow health
- SPF record accuracy
- DKIM signing and alignment
- DMARC policy alignment
- Gateway routing behaviour
- Domain reputation and blacklist status
All checks passed.
Resolution
Because the root causes were recipient-side, remediation required coordination with external IT teams. We provided the client with:
- Technical escalation summaries for each affected domain
- TLS certificate error evidence
- Diagnostic logs and message trace findings
- Gateway outbound IP details for allowlisting
- Recommended escalation wording for faster handoff
Recipient administrators then renewed certificates, corrected chain installation, and updated connector allowlists.
Outcome
After recipient-side remediation:
- Delivery was fully restored
- Bounce-back errors stopped
- Microsoft 365 mail flow normalised
- No sender-side configuration changes were required
- Business communications resumed without disruption
Key Takeaway
Not all Microsoft 365 delivery failures originate in the sender tenant. Recipient TLS certificate errors, broken trust chains, and restrictive inbound connector policies can block otherwise healthy mail flow, even when SPF, DKIM, and DMARC are correctly configured.
A structured, evidence-led investigation prevents unnecessary configuration changes and speeds up resolution.
Need Help with Microsoft 365 Email Issues?
If your business is seeing bounce-backs, NDRs, or Office 365 deliverability problems, our team can help with rapid diagnostics across authentication, routing, encryption, and gateway security.
Contact us for Microsoft 365 setup in Wandsworth, email setup in Wandsworth, and remote IT support.