Thunderbird Cannot Connect to outlook.office365.com – Step-by-Step Troubleshooting

Written By Tej Pratap Shukla

Approved By Anuraag Singh

Modified On November 26th, 2025

Reading Time 8 Min Read

If you use Thunderbird to access your Office 365 (Microsoft 365) mailbox, then at one time of time you face these types of errors:

“Thunderbird cannot connect to outlook.office365.com”
“Login to server outlook.office365.com failed Thunderbird”
“Login to server outlook.office365.com with username failed Thunderbird”

I’ve faced these issues several times in my own setups, and the root cause I found is always related to IMAP authentication failed, outdated Thunderbird settings, or version.

Today, through this guide, I will share all my experiences and step-by-step techniques to resolve the issue in minutes.

Why Microsoft 365 Doesn’t Officially Support Thunderbird?

Before moving to the how-to, I have one question for you: Do you know Microsoft doesn’t support Thunderbird for integration? Instead, it supports only native protocols:

Exchange Online
Outlook Desktop
Outlook Web
Outlook Mobile

Thunderbird depends on IMAP + SMTP, which Microsoft considers legacy protocols. This means, if Microsoft changes authentication rules, Thunderbird is unable to connect.

Despite that, Thunderbird works flawlessly as long as OAuth2 and IMAP are set up properly.

Before You Troubleshoot Thunderbird Cannot Connect to outlook.office365.com: Ensure Offline Backup

If you’re facing this issue first time, then one thing let me clear is that email access can be disrupted for hours or even days, depending on the cause. While resolving the issue, you can’t access your data, and even syncing is also unable.

During this downtime, you can avoid any risk of missed emails or data loss by creating an offline backup of your IMAP mailbox.

A secure and quick way to do this is by using a dedicated utility. For the same reason, I personally prefer the SysTools IMAP Backup Solution, which can securely export the mailbox data to multiple formats such as PST, MBOX, EML, PDF, and MSG.

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This is especially helpful in scenarios like:

OAuth2 login loops
Login to server outlook.office365.com failed Thunderbird errors
Temporary Microsoft 365 throttling
Archive IMAP emails
Conditional Access policies blocking IMAP
Corrupted Thunderbird profiles

By having a complete offline copy of your emails, you ensure data continuity, even if you can’t access the Thunderbird service. So, download this solution now for free, and secure your data.

Why Thunderbird Cannot Connect to outlook.office365.com

Thunderbird uses IMAP/SMTP to connect to the Office 365 mailbox. However, Microsoft has changed several authentication requirements as per the latest update. According to them, the most common causes are:

Microsoft has officially disabled Basic Authentication, i.e. username and password, for Office 365. So if Thunderbird is using basic auth, the connection will fail and result in login to server outlook.office365.com failed Thunderbird error.
Office 365 now requires modern authentication (OAuth2). Many older Thunderbird setups still use normal password authentication.
Wrong IMAP/SMTP settings\
If you’ve turned on Multi-Factor Authentication, you will need to use an App Password instead of your regular password.
Disabled IMAP in Microsoft 365 Admin Center

After learning the causes behind it, let me explain how to resolve Thunderbird cannot connect to outlook.office365.com.

How to Fix Login to Server outlook.office365.com Failed Thunderbird Issue

Below are my step-by-step fixes, which I used to troubleshoot the error quickly and effectively:

#Fix 1. Update Thunderbird Authentication to OAuth2

As we learned in the causes section, also using OAuth2 instead of legacy authentication will resolve the issue 90% of the time. Here are the instructions to do so:

Open Thunderbird
Go to Account Settings
Select the account
Click Server Settings
Change:
- Authentication Method: OAuth2
- Connection Security: SSL/TLS
Now go to Outgoing Server (SMTP)
Select your Office 365 SMTP entry
Change:
- Authentication Method: OAuth2
- Connection Security: STARTTLS

Now, verify the IMAP/SMTP settings:

IMAP (Incoming)

Server: outlook.office365.com
Port: 993
SSL/TLS: Yes
Authentication: OAuth2
Username: Full email address

SMTP (Outgoing)

Server: smtp.office365.com
Port: 587
STARTTLS: Yes
Authentication: OAuth2
Username: Full email address

After saving, Thunderbird will open a Microsoft login pop-up. Log in normally and approve the request. But, if this pop-up doesn’t appear, there’s an issue with old Thunderbird credentials. Don’t worry, use the next fix.

#Fix 2. Remove Old Saved Passwords

Thunderbird often stores old Basic Auth passwords, which cause “login to server outlook.office365.com with username failed Thunderbird.” To resolve it:

Open Thunderbird
Go to Settings
Select Privacy & Security
Scroll to Saved Passwords
Click Remove All related to:
- outlook.office365.com
- smtp.office365.com
Restart Thunderbird

Now reconnect using OAuth2 again.

#Fix 3. Enable IMAP in Microsoft 365 Admin Center

If IMAP is disabled, Thunderbird cannot connect, and shows Thunderbird cannot connect to outlook.office365.com error. Here’s how to resolve it:

#Option A: Enable globally

Log in to Microsoft 365 Admin Center
Navigate to:
Settings > Org Settings > Modern Authentication
Make sure:
- IMAP is enabled
- SMTP AUTH (if required) is enabled

#Option B: Enable per user

Go to Active Users
Select the user
Go to Mail > Manage email apps
Check:
- IMAP
- Authenticated SMTP (if needed)
- Restart Thunderbird.

#Fix 4. Use an App Password

If MFA is enabled and OAuth2 is not supported, leverage app password. So, in some Microsoft tenants, OAuth2 gets restricted. In that case, you can fix the Thunderbird cannot connect to outlook.office365.com issue by generating an App Password. For the same:

Log in to your Microsoft account
Go to Security Info
Select Add Method > App Password
Generate a password
Enter this password in Thunderbird instead of your usual one

This quickly resolves:

#Fix 5. Update Thunderbird to the Latest Version

Older versions of Thunderbird do not support the latest Office 365 authentication flows. So:

Go to Help > About Thunderbird
Update to the latest version
Retry the connection

Updating fixed everything immediately.

#Fix 6. Check Firewall / Antivirus Blocking IMAP Ports

Some security tools block port 993 (IMAP) or 587 (SMTP).

You should verify:

Port 993 is open
Port 587 is open
Thunderbird is added to the allowed applications

#Fix 7. Recreate the Thunderbird Account

If nothing else works, recreating the account without deleting local data solves corrupted profile issues. To do so:

Go to Account Settings
Click Account Actions > Remove IMAP Account
Restart Thunderbird
Re-add the account
Select IMAP
Choose OAuth2 during setup

This revolves IMAP folder is not properly syncing, and Thunderbird will automatically configure all folders again.

Alternative Ways to Access Office 365 if Thunderbird Stops Working

If you need urgent access, even when Thunderbird Cannot Connect to outlook.office365.com, these alternatives work instantly:

Outlook Web Access (OWA)
Windows Mail or macOS Mail
New Outlook App (Free)

What to Do When the OAuth2 Login Popup Does Not Appear

This is a very common issue, and you can fix it by:

#1. Remove stored OAuth cookies

Open Settings.
Select the Privacy option.
Now, click on Manage Cookies and remove them.

#2. Delete authentication tokens

logins.json
signons.sqlite

#3. Disable extensions temporarily

#4. Start Thunderbird in Troubleshoot Mode

Go to Help.
Click on Troubleshoot Mode
Retry account setup.

These quick fixes quickly resolve the missing OAuth pop-up issues.

Tenant-Level Security Policies That Break Thunderbird

Azure AD Conditional Access Policies can block Thunderbird silently, and leads to Thunderbird cannot connect to outlook.office365.com issue. Examples include:

Blocking “legacy” apps
Location-based restrictions
MFA requirements without App Password support

These policies generate authentication failures, like: login to server outlook.office365.com failed Thunderbird.

Common Mistakes to Avoid

These are the top mistakes that users accidentally make:

#Mistake 1. Choosing POP instead of IMAP causes an IMAP server timeout issue.
#Mistake 2. Wrong username format, as Office 365 requires a full email address.
#Mistake 3. Only changing IMAP authentication. Both IMAP and SMTP must use OAuth2.
#Mistake 4. Not removing saved passwords.

Author’s Verdict

As we discussed, the error “Thunderbird cannot connect to outlook.office365.com” is always a configuration or authentication mismatch. If you follow the steps I outlined, you should be able to reconnect quickly and resolve login to server outlook.office365.com failed Thunderbird.