Send & Receive Issues

How to Troubleshoot Slow or Failed Thunderbird SMTP with Microsoft 365

Chanuka Geekiyanage Oct 10, 2026 870 views

Question details

Users experience delayed or occasionally failing sent emails in Thunderbird when using a Microsoft 365 Business Standard account.

How to Troubleshoot Slow or Failed Thunderbird SMTP with Microsoft 365

Product: Thunderbird & Microsoft 365
Device & OS: Windows and Mac
Scenario: Sending emails via the Mozilla Thunderbird desktop client connected to a Microsoft 365 Business Standard email account.
Observed behavior: SMTP messages are delayed by several seconds or fail to send entirely, and this problem persists consistently across multiple devices and operating systems.

Before you start

Verify that your internet connection is stable and that you are using the correct SMTP settings (smtp.office365.com, Port 587, STARTTLS) before concluding it is a server-side issue.

Solution 1Recommended

Create a Service Request in the Microsoft 365 Admin Center

Since the SMTP delay and failure issue persists across multiple operating systems (Windows and Mac), it is likely a server-side configuration or routing problem that requires investigation by Microsoft support engineers.

Local email client troubleshooting is often insufficient for cross-platform SMTP failures. When issues replicate on both Windows and Mac environments, they usually stem from Microsoft 365 server backend delays or routing blocks.

Sign in to Admin Center

Navigate to the Microsoft 365 admin center in your web browser and sign in using your administrator credentials.

Navigate to Support

On the left-hand navigation menu, click on 'Support' and then select 'Help & support' to open the virtual assistant.

Search for your issue

Briefly describe the Thunderbird SMTP delay or failure issue in the search bar and press Enter to view automated diagnostic tools.

Contact support engineer

If the suggested diagnostic articles do not resolve the issue, select the 'Contact support' option to open a formal service request and allow a Microsoft engineer to review your server logs.

Create a Service Request in the Microsoft 365 Admin Center

Admin Access Required: You must have administrator privileges for your Microsoft 365 Business Standard tenant to create a service request. If you are a standard user, contact your IT department.

Solution 2

Verify Thunderbird SMTP Configuration

Ensure your outgoing server settings exactly match Microsoft 365 requirements to rule out local misconfigurations.

Free Microsoft Office alternative

Switch to WPS Office for a Lightweight Productivity Experience

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1. Download the software: Visit the official WPS Office website and download the free installer for your Windows or Mac computer.
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3. Open your files: Double-click any existing Microsoft Word, Excel, or PowerPoint files to open and edit them seamlessly in WPS Office.

Easily attach and send documents via your default email client (like Thunderbird) directly from the WPS Office interface.Fully compatible with all standard Microsoft Office formats, including .docx, .xlsx, and .pptx.Lightweight and completely free alternative to expensive Microsoft 365 subscriptions.Intuitive, tabbed user interface for seamless and familiar document migration.

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Frequently Asked Questions

What are the correct SMTP settings for Microsoft 365 in Thunderbird?

The correct settings are: Server Name: smtp.office365.com, Port: 587, Connection Security: STARTTLS, and Authentication Method: OAuth2.

Why are my emails delayed when sending through Microsoft 365?

Email delays can occur due to server-side routing issues, temporary service degradation in Microsoft's infrastructure, or strict spam-filtering protocols. If settings are correct and the issue spans multiple devices, it usually requires Microsoft support intervention.

Does Thunderbird support modern authentication for Microsoft 365 accounts?

Yes, Microsoft 365 now requires modern authentication (OAuth2) for all connections. Thunderbird fully supports OAuth2 for IMAP, POP3, and SMTP connections with Microsoft 365 accounts natively.

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