The vulnerability allows attackers to gain control of all the data stored within the mails received via the Mail app on iOS devices. Apple has recently become aware of the security problem, but the vulnerability may have been exploited since early 2018. A fix will soon be released, and, until then, iPhone and iPad users are advised to deactivate the Mail app and use other solutions like Gmail.
https://www.notebookcheck.net/Apple-s-iPhone-and-iPad-devices-have-been-plagued-by-serious-vulnerabilities-since-2018-security-analyst-reports.462299.0.html