If you want to connect Thunderbird to Office 365 reliably, you must use the correct server settings and modern authentication (OAuth2). Microsoft no longer allows legacy basic authentication for many accounts, so older Thunderbird installs might fail. This guide walks you through required steps, IMAP/SMTP details, admin checks, and common fixes — all in a compact, practical format.
1. Update Thunderbird & Prepare
Always start by updating Thunderbird to the latest release. Newer Thunderbird versions include OAuth2 support and fixes for Microsoft 365 integration. Go to Help → About Thunderbird and apply updates. Restart the app after updating.
2. Official IMAP and SMTP Settings
Incoming (IMAP)
Server: outlook.office365.com Port: 993 Security: SSL/TLS Authentication: OAuth2
Outgoing (SMTP)
Server: smtp.office365.com Port: 587 Security: STARTTLS Authentication: OAuth2
3. Add or Re-add Your Account (Auto-Config)
Remove any broken/old account configs and re-add your Office 365 email. Use Thunderbird’s automatic setup — it will query server settings and, if compatible, present the Microsoft OAuth2 login window. When prompted, sign in and accept permissions. If your organization uses Multi-Factor Authentication (MFA), complete that flow as well.
4. Modern Authentication & Admin Settings
If OAuth2 doesn’t appear, ask your Microsoft 365 admin to enable Modern Authentication for your tenant. Conditional Access policies or disabled IMAP at the tenant level can block third-party clients. Admins should verify IMAP protocols and allow legacy clients only if necessary (not recommended).
5. Common Problems & Quick Fixes
- Repeated password prompts: Delete saved passwords (Preferences → Privacy & Security → Saved Passwords) and re-authenticate with OAuth2.
- Login failed: Confirm correct ports and that OAuth2 is selected, not “Normal password”.
- No OAuth popup: Re-add account with automatic setup or clear cached credentials.
- Organization blocks: Check conditional access — your admin may whitelist approved clients only.
6. Best Practices
Prefer IMAP over POP for synced mail across devices. Keep Thunderbird and your OS updated. Avoid storing plain passwords: always use OAuth2 or app passwords only when directed by admin policies. Regularly review your mailbox size and archive old messages to keep Thunderbird responsive.