How many email clients support border-radius?

It works in 16 of 21, partial in 4, not supported in Outlook Classic.

Does border-radius work in AOL Mail?

border-radius is partly supported in AOL Mail. Closest to Outlook Classic: AOL Mail, which agrees on 206 of the 298 features both are tested on.

border-radius in Outlook Classic

Not supported border-radius is not supported in Outlook Classic.

Works in 16 of 21 · partial in 4 · not supported in 1

Which clients

Works: Gmail, Gmail Android, Gmail iOS, Outlook 365, Outlook (New), Outlook iOS, Outlook Android, Outlook for Mac, Apple Mail, Apple Mail iOS, Samsung Mail, Thunderbird, HEY Mail, Proton Mail, Fastmail, Superhuman
Partial: Yahoo Mail, Yahoo Mail Android, Yahoo Mail iOS, AOL Mail
Not supported: Outlook Classic

Your reader in Outlook Classic sees the email without border-radius, even if it looks right where you tested it.

The fix for Outlook Classic

Use VML to render rounded buttons in Outlook

after

18 lines

<!--[if mso]>
<v:roundrect xmlns:v="urn:schemas-microsoft-com:vml"
  href="https://example.com"
  style="height:44px; v-text-anchor:middle; width:200px;"
  arcsize="14%" strokecolor="#6d28d9" fillcolor="#6d28d9">
  <w:anchorlock/>
  <center style="color:#fff; font-family:Arial,sans-serif;
    font-size:14px; font-weight:bold;">Click Here</center>
</v:roundrect>
<![endif]-->
<!--[if !mso]><! -->
<a href="https://example.com"
  style="background-color: #6d28d9; color: #fff;
         padding: 12px 32px; border-radius: 6px;
         text-decoration: none; display: inline-block;">
  Click Here
</a>
<!-- <![endif]-->

Show all 18 lines

Show the original

before

6 lines

<a href="https://example.com"
  style="background-color: #6d28d9; color: #fff;
         padding: 12px 32px; border-radius: 6px;
         text-decoration: none; display: inline-block;">
  Click Here
</a>

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What Outlook Classic does with border-radius

The Emailens probe for border-radius on 3 real installs counted as Outlook Classic, captured 14 Oct 2026. It shows the problem, not the fix. Where border-radius is supported it shows: rounded corners.

The Emailens probe for border-radius on a real Outlook 2019 (Windows) — Outlook Classic

The Emailens probe for border-radius on a real Outlook 2016 (Windows) — Outlook Classic

Every account includes one free real-device check in classic Outlook, Outlook for Mac and Apple Mail, after you verify your email and run one preview, while this month's free capacity lasts. How real-device checks work →

caniemail record, tested upstream 9 Mar 2021

Round corners can be used in VML with the RoundRect element. See buttons.cm and VML documentation.

Your next question

Every feature in Outlook Classic · border-radius in every client

Questions

How many email clients support border-radius?: It works in 16 of 21, partial in 4, not supported in Outlook Classic.
What do I use instead of border-radius in Outlook Classic?: Use VML to render rounded buttons in Outlook.
Does border-radius work in AOL Mail?: border-radius is partly supported in AOL Mail. Closest to Outlook Classic: AOL Mail, which agrees on 206 of the 298 features both are tested on.

To see what else in your email Outlook Classic does not support, paste the whole email.

Check your email, free

No signup · Five clients free, Outlook Classic included · All 21 with an account

The checker needs a computer screen.

Real captures 14 Oct 2026 on 10 installs · caniemail (MIT) via @emailens/engine 0.18.22 · tested upstream 9 Mar 2021 · data last changed 10 Nov 2026 · Outlook (New) cells are the engine's mapping