A memory consumption issue was addressed with improved memory handling. This issue is fixed in iOS 13.5 and iPadOS 13.5, iOS 12.4.7, watchOS 6.2.5, watchOS 5.3.7. Processing a maliciously crafted mail message may lead to heap corruption.
https://www.tenable.com/blog/multiple-zero-day-vulnerabilities-in-ios-mail-app-exploited-in-the-wild
https://support.apple.com/HT211176
https://support.apple.com/HT211175