Microsoft Outlook vs Thunderbird
Thunderbird is the free, open source mail client; Outlook is free with ads, and ad-free through a Microsoft 365 subscription.
Pick Microsoft Outlook if…
Pick Outlook if you already use Microsoft services or want the same app on Windows, Mac, web and mobile, and don't mind ads or will pay for Microsoft 365. It is also built into Windows for free.
Pick Thunderbird if…
Pick Thunderbird if you want a free, ad-free, open source client for Windows, macOS or Linux, with mail, calendars and contacts in one app. Choose it if you prefer software funded by donations rather than ads.
Price
- Freemium
- Free
- Open source
Starting price
Free plan
Open source
Works on
| Feature | Microsoft's email and calendar app, with an ad-supported free version and ad-free Microsoft 365 plans. | A free, open source email app that combines mail, calendars and contacts, funded by donations. |
|---|---|---|
| Price |
|
|
| Starting price | Ad-free with a Microsoft 365 subscription, starting at $1.99 / month in the US | — |
| Free plan | Yes | Yes |
| Open source | No | Yes |
| Works on | Web, Windows, macOS, iOS, Android | Windows, macOS, Linux, Android |
| Try Microsoft Outlook (opens the Microsoft Outlook website in a new tab) | Try Thunderbird (opens the Thunderbird website in a new tab) |
Microsoft describes a free version of Outlook for personal use, and says Outlook is built into Windows for free with no subscription needed. Its support page says ads appear every time you use the free version, and that a Microsoft 365 subscription gives an ad-free experience, starting at $1.99 / month in the US.
The ad-free benefit is for consumer subscriptions. Microsoft's support pages say business plans such as Microsoft 365 Business Standard don't qualify for ad-free Outlook.com. Microsoft 365 Basic is described as including ad-free Outlook and 100 GB of cloud storage shared with OneDrive.
Thunderbird is free, which its site calls 'Free forever', and is funded by user donations. It offers a unified inbox or separate account management, plus calendars, contacts, tags, filters and extensions. The site says it is open source under MPL 2.0; the Android app is Apache-2.0 licensed and based on K-9 Mail.
Thunderbird's download page lists Windows, macOS, Linux and Android. Microsoft's pages cover Outlook on Windows, Mac, web and mobile.
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Last updated 16 October 2026. How we make money