In today’s Microsoft 365 Admin learning journey, we explored how emails move inside and outside the organization and how to troubleshoot delivery issues.

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📌 1. Understanding Mail Flow in Exchange Online

Mail flow refers to how email messages travel:

• Internal → Internal
• Internal → External
• External → Internal

All mail flow configurations are managed in:

🔗 https://admin.exchange.microsoft.com

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📌 2. Accepted Domains

Accepted Domains define which domains your organization can receive email for.

🔹 Types of Accepted Domains

1️⃣ Authoritative Domain

• Most common type
• Exchange Online hosts all mailboxes
• If recipient not found → NDR generated

2️⃣ Internal Relay

• Used in Hybrid environments
• If mailbox not found → email forwarded to another system

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📌 3. Mail Flow Rules (Transport Rules)

Mail flow rules allow administrators to:

• Block specific keywords
• Add disclaimers
• Apply encryption
• Redirect messages
• Restrict attachment types
• Send copies to compliance mailbox

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🛠️ Practical Lab – Create a Mail Flow Rule

🎯 Scenario:

Block emails containing “Confidential Test”

🔹 Steps:

1. Login to Exchange Admin Center
2. Navigate to Mail Flow → Rules
3. Click + Add a Rule
4. Name the rule: Block Confidential Test
5. Under “Apply this rule if”
   → Select Subject or body includes
   → Add keyword: Confidential Test
6. Under “Do the following”
   → Select Reject the message
   → Add explanation text
7. Click Save

✅ Rule is now active.

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📌 4. Message Trace (Email Troubleshooting Tool)

Message Trace helps admins:

• Track delayed emails
• Identify bounced emails
• Check spam filtering status
• Confirm successful delivery

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🔹 Steps to Run Message Trace

1. Go to Exchange Admin Center
2. Navigate to Mail Flow → Message Trace
3. Click Start a trace
4. Enter:
   • Sender email
   • Recipient email
   • Date range
5. Click Search

You can see:

• Delivered
• Failed
• Pending
• Quarantined

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📌 5. Email Authentication (SPF, DKIM, DMARC)

These DNS records protect your domain from spoofing.

🔹 SPF (Sender Policy Framework)

• Defines authorized sending servers

🔹 DKIM (DomainKeys Identified Mail)

• Digitally signs outgoing email

🔹 DMARC

• Defines what to do if SPF/DKIM fails

These improve:

• Email deliverability
• Domain reputation
• Security posture

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🎯 Real-World Admin Scenario

User Complaint:

“My client didn’t receive my email.”

Troubleshooting Checklist:

✅ Run Message Trace
✅ Check Quarantine
✅ Verify Mail Flow Rules
✅ Review Connectors
✅ Validate SPF/DKIM/DMARC
✅ Check if recipient server rejected