How many email clients support opacity?

It works in 19 of 21, not supported in Outlook Classic and AOL Mail.

Does opacity work in Fastmail?

opacity works in Fastmail. Closest to HEY Mail: Fastmail, which agrees on 173 of the 235 features both are tested on.

opacity in HEY Mail

Works opacity works in HEY Mail.

Works in 19 of 21 · not supported in 2

Which clients

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

The fix for the clients that need it

Needed in 2 of 21: Outlook Classic and AOL Mail.

Replace opacity with solid color equivalents

instead

5 lines

.overlay {
  /* Use a semi-transparent color instead of opacity */
  background-color: #808080;
  /* Or for modern clients: rgba(0, 0, 0, 0.5) */
}

Show the original

before

4 lines

.overlay {
  background-color: #000;
  opacity: 0.5;
}

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Your next question

Every feature in HEY Mail · opacity in every client

Questions

How many email clients support opacity?: It works in 19 of 21, not supported in Outlook Classic and AOL Mail.
Does opacity work in Fastmail?: opacity works in Fastmail. Closest to HEY Mail: Fastmail, which agrees on 173 of the 235 features both are tested on.

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caniemail (MIT) via @emailens/engine 0.18.22 · tested upstream 28 Feb 2019 · data last changed 10 Nov 2026 · Outlook (New) cells are the engine's mapping