max-block-size
CSS property · 6 supported, 0 partial, 11 unsupported across 21 clients
As of Sep 2, 2026, max-block-size fails or degrades in 15 of 21 email clients, treat it as progressive enhancement, never structure.
- 1/7 webmail, 2/6 desktop, 3/8 mobile clients support max-block-size fully.
- Unsupported in Gmail, Gmail Android, Gmail iOS, Outlook 365 and 7 more.
- Outlook Classic (Word engine, supported to at least 2029) does not fully support max-block-size, and its replacement changes little here; see the deprecation diff.
- Across all clients, only 6 of 298 tracked HTML/CSS features render identically everywhere.
Source: caniemail.com (MIT), via the Emailens engine. Last tested upstream Sep 1, 2022, synced by Emailens Sep 2, 2026.
Client Support
| Client | Category | Engine | Support |
|---|---|---|---|
| Gmail | webmail | Gmail Web | Unsupported |
| Gmail Android | mobile | Gmail Mobile | Unsupported |
| Gmail iOS | mobile | Gmail Mobile | Unsupported |
| Outlook 365 | webmail | Outlook Web | Unsupported |
| Outlook (New) | desktop | Outlook Web | Unsupported |
| Outlook Classic | desktop | Microsoft Word | Unsupported |
| Outlook iOS | mobile | Outlook Mobile | Supported |
| Outlook Android | mobile | Outlook Mobile | Unsupported |
| Outlook for Mac | desktop | WebKit | Supported |
| Apple Mail | desktop | WebKit | Supported |
| Apple Mail iOS | mobile | WebKit | Supported |
| Yahoo Mail | webmail | Yahoo | Unsupported |
| Yahoo Mail Android | mobile | Yahoo | Unsupported |
| Yahoo Mail iOS | mobile | Yahoo | Unsupported |
| Samsung Mail | mobile | Samsung | Supported |
| Thunderbird | desktop | Gecko | Unknown |
| HEY Mail | webmail | WebKit | Unknown |
| Proton Mail | webmail | Proton | Supported |
| AOL Mail | webmail | AOL | Unsupported |
| Fastmail | webmail | Fastmail | Unknown |
| Superhuman | desktop | Blink | Unknown |
Client-by-client behaviour for max-block-size
Fully supports max-block-size (6): Outlook iOS, Outlook for Mac, Apple Mail, Apple Mail iOS, Samsung Mail, Proton Mail.
No support (11): Gmail, Gmail Android, Gmail iOS, Outlook 365, Outlook (New), Outlook Classic, Outlook Android, Yahoo Mail, Yahoo Mail Android, Yahoo Mail iOS, AOL Mail. Plan fallbacks for these clients before relying on max-block-size in production sends.
Behaviour unverified in: Thunderbird, HEY Mail, Fastmail, Superhuman.
Fixes & Workarounds
Gmail
Unsupported"max-block-size" is not supported in this email client.
Gmail Android
Unsupported"max-block-size" is not supported in this email client.
Gmail iOS
Unsupported"max-block-size" is not supported in this email client.
Outlook 365
Unsupported"max-block-size" is not supported in this email client.
Outlook (New)
Unsupported"max-block-size" is not supported in this email client.
Outlook Classic
Unsupported"max-block-size" is not supported in this email client.
Outlook Android
Unsupported"max-block-size" is not supported in this email client.
Yahoo Mail
Unsupported"max-block-size" is not supported in this email client.
Yahoo Mail Android
Unsupported"max-block-size" is not supported in this email client.
Yahoo Mail iOS
Unsupported"max-block-size" is not supported in this email client.
AOL Mail
Unsupported"max-block-size" is not supported in this email client.
Related Features
Check your own HTML for max-block-size
Paste your own, or run the sample below. Analyzed in this page, with no signup and nothing stored.
Test your max-block-size layout across 21 email clients
Preview rendering, catch CSS issues, and see how max-block-size displays across Outlook, Gmail, and Apple Mail. Free, no account needed.
Instant live preview · No signup required · 21 email engines
Last tested upstream: · Last synced by Emailens:
caniemail.com (MIT) records Sep 1, 2022 as the last time max-block-size was tested in a real client, 4 years before this sync. Syncing copies that result forward, it does not re-test it. Carried in @emailens/engine data v0.12.2.