How many email clients support ?

It works in 10 of 21, partial in 4, not supported in 7.

Does work in Apple Mail?

works in Apple Mail. Closest to Fastmail: Apple Mail, which agrees on 192 of the 235 features both are tested on.

<form> in Fastmail

Not supported <form> is not supported in Fastmail.

Works in 10 of 21 · partial in 4 · not supported in 7

Which clients

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

Your reader in Fastmail sees the email without <form>, even if it looks right where you tested it.

The fix for Fastmail

Replace embedded form with a link to a hosted form

after

13 lines

<table role="presentation" cellpadding="0" cellspacing="0">
  <tr>
    <td style="background-color: #6d28d9; border-radius: 6px;
               padding: 12px 32px;">
      <a href="https://example.com/subscribe"
        style="color: #fff; text-decoration: none;
               font-family: Arial, sans-serif;
               font-weight: bold;">
        Subscribe Now
      </a>
    </td>
  </tr>
</table>

Show the original

before

4 lines

<form action="/subscribe" method="POST">
  <input type="email" placeholder="Email" />
  <button type="submit">Subscribe</button>
</form>

Test this fix, free

Fastmail opens with a free account; the checker shows five clients without one.

The checker needs a computer screen.

A free account opens all 21 clients and keeps your checks for 7 days. Create a free account →

What Fastmail does with <form>

@emailens/engine 0.18.22 on the original code

Fastmail strips form elements.

Your next question

Every feature in Fastmail · <form> in every client

Questions

How many email clients support <form>?: It works in 10 of 21, partial in 4, not supported in 7.
What do I use instead of <form> in Fastmail?: Replace embedded form with a link to a hosted form.
Does <form> work in Apple Mail?: <form> works in Apple Mail. Closest to Fastmail: Apple Mail, which agrees on 192 of the 235 features both are tested on.

To see what else in your email Fastmail 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.

caniemail (MIT) via @emailens/engine 0.18.22 · tested upstream 10 Sep 2019 · data last changed 10 Nov 2026 · Outlook (New) cells are the engine's mapping