Re:Google is testing expiring emails in the new Gmail
by JonathonW on Friday April 13, 2018 @01:22PM
(#16831999)
If you're looking for an "open ecosystem" option, Mail on both iOS and Mac (as well as a bunch of clients elsewhere, including Outlook and Thunderbird) supports S/MIME encryption and signing out of the box.
Of course, no one uses it or even realizes it's there because S/MIME is terrible.
Of course, hopefully PGP, and not some custom, private API like iMessage.