hidden attribute

Supported

• Apple Mail (macOS)
• Apple Mail (iOS)
• Samsung Email (Android)
• Mozilla Thunderbird (macOS)
• SFR (Desktop Webmail)
• SFR (iOS)
• SFR (Android)

Partially supported

• Mail.ru (Desktop Webmail)

Not supported

• Gmail (Desktop Webmail)
• Gmail (iOS)
• Gmail (Android)
• Gmail (Mobile Webmail)
• Outlook (Windows)
• Outlook (Windows Mail)
• Outlook (macOS)
• Outlook (Outlook.com)
• Outlook (iOS)
• Outlook (Android)
• Yahoo! Mail (Desktop Webmail)
• Yahoo! Mail (iOS)
• Yahoo! Mail (Android)
• AOL (Desktop Webmail)
• GMX (Desktop Webmail)
• WEB.DE (Desktop Webmail)