Microsoft Outlook vs Thunderbird

Fit:

Migration notes

Local alternative for email, calendars, contacts, tasks, and multi-account organization. Reconnect mail accounts using supported authentication; Microsoft-hosted accounts may require OAuth, SMTP enablement, and administrator consent. Full Exchange/calendar or Microsoft 365 suite parity is not asserted. Existing subscription storage/security benefits are separate from the client.

Sources: www.thunderbird.net, support.mozilla.org, www.microsoft.com · checked 2026-10-21

Microsoft OutlookPaid incumbent

Entry priceUSD 19.99 / account / year

Pricing modelflat, billed annual

OwnerMicrosoft Corporation

Microsoft 365 Basic, US annual price; includes ad-free Outlook and 100 GB mailbox. Free Outlook also exists; this record represents paid subscription benefits, not an OSS edition. Source

Microsoft Outlook plans reviewed
Microsoft 365 Basic	entry	USD 1.99 / person / mo
Microsoft 365 Apps for business	business	USD 120 / user / yr
Microsoft 365 E7	enterprise	USD 1,188 / user / yr

ThunderbirdOpen-source alternative

Fit for Microsoft Outlook

License

Mozilla Public License 2.0 Permissions of this weak copyleft license are conditioned on making available source code of licensed files and modifications of those files under the same license (or in certain cases, one of the GNU licenses). Copyright and license notices must be preserved. Contributors provide an express grant of patent rights. However, a larger work using the licensed work may be distributed under different terms and without source code for files added in the larger work. May Commercial useModificationDistributionPatent usePrivate use Must Disclose sourceLicense and copyright noticeSame license (file) No LiabilityTrademark useWarranty Summary from GitHub's choosealicense.com

Paid-feature tier A

Self-host

Paid-feature limitsNone established

Thunderbird plans reviewed
Thunderbird Desktop	free desktop/core	Free

What you may give up

No paid-feature limits established for this edition.

Requires an existing email account/mail service; the desktop client is not an IMAP/SMTP server or hosted webmail.
Thundermail is a separate optional paid mail-hosting service in beta/waitlist; it does not paywall the desktop client.

Tier A: Desktop client operated locally on Windows, Linux, and macOS; A describes the freely available client functionality, not server hosting. Official pages call it free forever and link MPL 2.0. Dependencies/other-licenses have separate notices; not every bundled file is MPL. Current developer docs and repository identify thunderbird/thunderbird-desktop as official, superseding use of the experimental Mozilla GitHub copy as canonical. License source

What running it takes

Installed Thunderbird binary

Hover or focus an icon for its name. Dashed means not established; struck through means no; grouped icons are alternatives.

Local desktop installation; no external SQL/cache/queue server required. Existing mailbox service needed for mail. Source build requirements deliberately excluded. Infrastructure and staff time are not priced here; this page does not compare total cost.