Send & Receive Issues

Fix Office 365 SMTP Email Authenticated But Not Delivered

Rana Garcia Oct 10, 2026 868 views

Question details

A custom application successfully authenticates and sends emails via Microsoft 365 SMTP, but the intended recipients never receive the messages.

How to Fix Office 365 SMTP Email Authenticated But Not Delivered

Product: Microsoft 365 Exchange Online
Device & OS: not provided
Scenario: Sending automated HTML emails through Office 365 SMTP using a Python script or similar application.
Observed behavior: The application reports successful authentication and delivery, but the emails disappear without reaching the recipient's inbox or junk email folder.

Before you start

Ensure you have Exchange Administrator privileges in your Microsoft 365 tenant, as you will need access to the Exchange Admin Center to run diagnostic message traces.

Solution 1Recommended

Run a Message Trace in the Exchange Admin Center

Use the Exchange Admin Center to see exactly where Microsoft 365 dropped or quarantined the message after accepting it from your application.

When an application successfully connects to the SMTP server, it only means the server accepted the message. A message trace reveals what happened during the internal routing process.

Access the Exchange Admin Center

Log in to the Microsoft 365 admin center, expand the left navigation menu, and click on 'Exchange' to open the Exchange Admin Center (EAC).

Navigate to Message Trace

In the left-hand menu of the EAC, go to 'Mail flow' and select 'Message trace'.

Start a New Trace

Click 'Start a trace'. Enter the specific sender address used by your Python application and define a time range (e.g., the last 24 hours). Click 'Search' to run the trace.

Analyze the Results

Review the trace results. Look at the status column to see if the message was marked as 'Failed', 'Quarantined', 'Filtered as Spam', or 'Delivered'.

Run a Message Trace in the Exchange Admin Center

No Trace Found?: If the message trace is completely empty, Microsoft 365 never received the email. Double-check your application's connection settings, firewall rules, and ensure it is genuinely reaching smtp.office365.com.

Solution 2

Verify SMTP Configuration and DNS Records

Ensure your application uses the exact connection settings required by Microsoft 365 and that your domain is authorized to send emails.

Solution 3

Check Tenant Sending Limits and Spam Policies

Microsoft 365 may silently block outbound emails if the sending mailbox exceeds limits or triggers anti-spam policies.

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Frequently Asked Questions

Why does my Python app say the email was sent successfully if it never arrived?

The 'sendmail' command in your application only confirms that the Microsoft 365 SMTP server accepted the message. It does not track what happens after Microsoft routes the email through its spam filters or when it reaches the final recipient's server.

Do I need an app password to send emails via Office 365 SMTP?

Yes, if your Microsoft 365 tenant enforces Multi-Factor Authentication (MFA) or Security Defaults, basic SMTP AUTH will fail. You must generate an App Password in your Microsoft account settings or configure an Office 365 SMTP relay connector instead.

How do SPF, DKIM, and DMARC affect my email delivery?

These DNS records prove to recipient servers that your domain officially authorized Microsoft 365 to send emails on its behalf. If they are missing or incorrect, recipient email servers (like Gmail or Yahoo) may silently drop the emails to prevent spoofing, leading to delivery failures with no bounce messages.

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