In reference to Litmus, the table below shows which email clients support, somewhat support, or don’t support forms:

Email Client Forms supported? Notes
Apple Mail Yes “GET” works as expected. “POST” opens up a landing page with a form instead of submitting from the email
iPhone Mail App Somewhat “GET” works as expected. “POST” does not submit.
Gmail Desktop Webmail Yes Submits with a popup alert.
Gmail (iOS) Somewhat Does not submit in iOS GANGA. “POST” input is also disabled.
Gmail Android Somewhat Only “GET” works, but opens up a landing page with a form instead of submitting from the email. With “POST”, the form visually appears, but does not allow user input.
Gmail Mobile Webmail Yes
Outlook on Windows No
Windows 10 Mail No
Outlook MacOS No The form visually appears and allows user input on Microsoft email addresses, but does not allow submission.
Outlook.com No Submits only when an email is opened in its own window, rather than submitting from inbox view. Upon submission, input attributes are prefixed with “x_” so it can cause issues with collecting data. Would recommend hiding for this client.
Outlook iOS Yes
Outlook Android Somewhat The form shows visually, but the keyboard doesn’t appear to allow users to input information so users would have to copy and paste text to input. Would recommend hiding for this client when a keyboard input is used but allow forms if using other inputs.
Yahoo! Desktop Webmail Yes Submits with a popup alert.
Yahoo! iOS No The form visually appears and allows user input, but does not allow submission.
Yahoo! Android Somewhat The form shows visually, but the keyboard doesn’t appear to allow users to input.
AOL Desktop Webmail Yes
AOL iOS No The form visually appears and allows user input, but does not allow submission.
AOL Android Yes
Samsung Mail App Somewhat The form shows visually, but the keyboard doesn’t appear to allow users to input information. Would recommend hiding for this client when a keyboard input is used but allow forms if using other inputs.