How to Fix the ‘Thunderbird Not Working with Office 365′ Error?

Written By Adom James
Updated on February 5th, 2026

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Introduction:Mozilla Thunderbird is the best desktop-based email service. This email client is a free open-source email client. Sometimes, the configuration could not be verified to Office 365 due to various issues. As a result, Thunderbird not working with Office 365 issue arises and users seek a reliable solution to resolve it. To ease your workload, we have researched the issue and found an appropriate solution. Most people choose Thunderbird Backup Tool for a hassle-free outcome. Let’s go sequentially and understand the reasons first.

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Like you, many users encounter the same situation when Mozilla Thunderbird works with Office 365. However, Thunderbird failed to connect to Office 365 due to various reasons. Let’s explore the different reasons for the situation. So, you can avoid and fix it.

Table of Contents

Reasons: Thunderbird is Not Working with Office 365

Here, read this section carefully to understand the different perspectives on the issue. So, you can avoid it from occurring in the future as well.

This could happen when you configure Thunderbird to Office 365 incorrectly.
If IMAP or SMTP is disabled in Office 365 account, Thunderbird cannot work with Office 365.
When you use the wrong Thunderbird authentication, the configuration with Office 365 will be a failure.
There are chances of server issues that constrain Thunderbird from working with Office 365.
Check whether you are using the latest version of Thunderbird instead of the outdated version.

Thus, these issues restrict you from connecting your Thunderbird email client to Office 365. Moving forward, we will discuss the problem along with its solution.

Solutions: Thunderbird Failed to Connect to Office 365

In this section, we have covered different problems. Also, the solution for the issue, the configuration could not be verified to Office 365.

Scenario 1. When User Authenticate But not Connected: IMAP Response in Thunderbird

The situation where authentication is done but users are still disconnected. In that scenario, IMAP responds with the two messages.

When the recent operation on the inbox is unsuccessful. The IMAP response as User is authenticated but not connected.
The outlook.office365.com server is disconnected. It could be because of server downtime or network issues.

Before we begin the solution, check whether your Thunderbird is in the updated or latest version. If your Thunderbird is up-to-date, visit the IPv6 settings and follow the below guidance.

Launch Mozilla Thunderbird Account.
Tap on the gear icon at the extreme top-left corner.
Now, tap on the General option from the left-top corner.
Select the Configuration Editor at the end.
Type the network.dns.dis in the dialogue box.
Choose the network.dns.disableIPv6 and press the toggle to change from false to true.
Next, close the Thunderbird application and restart it.

Scenario 2. Failure: Thunderbird not Working with Office 365

Sometimes, when configuring Thunderbird to Office 365, you might have received the message – Log in failure: unknown username or incorrect password. So, when you add Thunderbird to Office 365, check for these details and change it accordingly.

In IMAP

Server name- outlook.office365.com.
Port- 993.
Encryption method- TLS.

In POP

Server name- outlook.office365.com.
Port- 995.
Encryption method- TLS.

In SMTP

Server name- smtp-mail.outlook.com.
Port- 587.
Encryption method- STARTTLS.

Well, the POP is a disabled default, you have to enable it. Now, follow the given steps to enable POP.

Tap on the Settings option >> View all Outlook settings option.
Now, click on the Mail tab >> Sync Email option.
In the Let devices and apps use POP, choose Yes.
Lastly, press the Save button.

Still, you cannot connect to Thunderbird to Office 365. You can read the below procedure.

Automated Software: Thunderbird Not Working with Office 365

The CloudMigration Thunderbird Email Backup Tool is the ultimate option when the above solution cannot resolve your problem. Unlike other solutions, it will transfer the Thunderbird data into the Office 365 email client. Also, the process will be efficient and effective. If your Thunderbird failed to connect to Office 365, transfer the data into multiple file formats including MBOX, PST, etc. Also, you can Backup Thunderbird Emails to External Hard Drive.

Steps to Resolve Thunderbird is not Working with Office 365

When Thunderbird failed work with Office 365 emails, follow the guide to resolve the issue.

Download the Thunderbird Email Backup Tool on your PC.
Enter the profile from which you want to migrate the data.
Pick the Thunderbird Folder from the tree structure mailbox. Press Next.
Next, select the Office 365 email client from the saving options menu.
Sign In to the Office 365 account and use software features if required.
Press on the Convert tab to migrate the Thunderbird data into Office 365.

Thus, these steps will assist you in migrating the Thunderbird data into Office 365 directly. So, you can access the Thunderbird data in Office 365 easily.

Summarizing

When you go through this blog, you will get the solution to fix the issue- “Thunderbird not working with Office 365”. However, we provide two suitable methods: manual and Professional. In case the manual solution didn’t work, try the Thunderbird Backup Tool to resolve your query. It will help you to complete the process without issues or complications.

Frequently Asked Questions

Q1. Why is Thunderbird not connecting to Office 365?

Ans. The common reasons why Thunderbird doesn’t connect to Office 365 are:
– Disabled IMAP
– Incorrect Server Settings
– Blocked authentication
– Outdated Thunderbird or system versions
– Microsoft security policies

Q2. Is Thunderbird compatible with Office 365?

Ans. Yes, Thunderbird is compatible with Office 365 and easily connects via IMAP and SMTP. However, it requires Modern authentication, app passwords, and other support as per the profile and connection.

Q3. What are the correct Office 365 IMAP settings for Thunderbird?

The correct IMAP settings are:
Server: outlook.office365.com, Port 993, Encryption: SSL/TLS
SMTP: smtp.office365.com, Port 587, Encryption: STARTTLS

Q4. How do I fix Office 365 login errors in Thunderbird?

To fix the Office 365 login errors, try to use OAuth, enable IMAP, use MFA, generate an app password, and clear the cache. Or, use the advanced Thunderbird Backup Tool for best results.

Q5. What should I do if Thunderbird suddenly stops working with Office 365?

If Thunderbird suddenly stops working, check updates, re-authenticate account, verify IMAP, confirm OAuth, or else get a dedicated Thunderbird Email Backup software.

Q6. How do I enable IMAP in Office 365 for Thunderbird?

Open the Microsoft 365 Admin Center and go to the user mailbox. Then, select your Mailbox Settings and in Email Apps, you can find IMAP. Ensure to toggle it on and save changes.

About The Author:

Adom James

I’m Adom james, a skilled content strategist with a focus on technical writing. With in-depth knowledge in Email Backup, Data Recovery, and Email Migration, I specialize in providing expert solutions and addressing technical challenges for both individuals and businesses, especially in the areas of Cloud Backup and seamless email transitions.