How different are Thunderbird and Outlook Classic?

They disagree on 185 of the 254 features verified in both clients, or 73%. 52 features render in both and are the safe intersection to build on.

What works in Thunderbird but not Outlook Classic?

149 features, including , , , , and 144 more. Each needs a fallback if the send reaches both clients.

Which engines are these two clients using?

Thunderbird renders with Gecko; Outlook Classic renders with Microsoft Word. Where two clients share an engine the divergence is usually configuration, and where they do not it is the engine itself.

Is this measured in the real clients?

No. These are documented support levels derived from caniemail.com (MIT-licensed), carried in @emailens/engine data version 0.12.2 and last synced Sep 2, 2026. That sync date is not a test date: caniemail's own per-feature tests run from Feb 28, 2019 to Jul 13, 2026. Anything described as a transform is applied per those documented rules rather than captured from a real client.

Thunderbird vs Outlook Classic: CSS Support Differences

Thunderbird renders with Gecko; Outlook Classic renders with Microsoft Word. The gap runs almost entirely one way: 149 features work in Thunderbird and break in the other, against 3 in the reverse direction. Build for the stricter one and the permissive one follows.

Support data derived from caniemail.com (MIT) - last synced Sep 2, 2026 - engine data v0.12.2

work in both

the safe intersection

149

only in Thunderbird

breaks in Outlook Classic

only in Outlook Classic

breaks in Thunderbird

partial on one side

renders, with caveats

Works in Thunderbird, breaks in Outlook Classic

Each of these needs a fallback if the send reaches both.

Works in Outlook Classic, breaks in Thunderbird

Partial on one side

These render in both, but one client applies caveats the other does not. They fail quietly, which makes them the expensive ones.

A further 17 features render in neither client. Not a difference between them, but the other half of the answer to what you can actually use, so the four counts above deliberately do not sum to 254.

44 of 298 features have no confident data point for at least one of these clients, so they are excluded from every count above rather than assumed either way.

Where the divergence sits

Two pairs with the same headline number can be different problems. For these two it concentrates in CSS Properties.

CSS Properties 98
HTML Elements 32
Pseudo-Classes & Selectors 24
HTML Attributes 10
CSS Functions 8
Pseudo-Elements 6

30 of these already have a fix

Each links to a copy-paste snippet that keeps the feature working in Outlook Classic without dropping it in Thunderbird.

Check your own HTML against both

Check your own HTML

Paste your own, or run the sample below. Analyzed in this page, with no signup and nothing stored.

Questions

How different are Thunderbird and Outlook Classic?: They disagree on 185 of the 254 features verified in both clients, or 73%. 52 features render in both and are the safe intersection to build on.
What works in Thunderbird but not Outlook Classic?: 149 features, including <abbr>, <acronym>, <audio>, <base>, <bdi> and 144 more. Each needs a fallback if the send reaches both clients.
Which engines are these two clients using?: Thunderbird renders with Gecko; Outlook Classic renders with Microsoft Word. Where two clients share an engine the divergence is usually configuration, and where they do not it is the engine itself.
Is this measured in the real clients?: No. These are documented support levels derived from caniemail.com (MIT-licensed), carried in @emailens/engine data version 0.12.2 and last synced Sep 2, 2026. That sync date is not a test date: caniemail's own per-feature tests run from Feb 28, 2019 to Jul 13, 2026. Anything described as a transform is applied per those documented rules rather than captured from a real client.

Zero-Friction Engine Preview

Test your email in Gmail, Outlook and Apple Mail

Preview rendering, catch CSS issues, and see what Outlook does to your VML. Free, no account. All 21 clients with one.

Instant live preview · No signup required · 21 email engines