Give Signal a Try
There are plenty of third-party messaging apps that boast end-to-end encryption. Like iMessage, there’s a good chance that you already use one of them (like WhatsApp). But if you’re looking for arguably the most secure and reliable messaging app for secret messages, Signal may be your best bet.
Among E2E apps, Signal boasts some of the strongest privacy and security protections. A favorite of investigative journalists and activists beating authoritarian surveillance, Signal stores so little data about its users that it’s been vouched-for by infamous NSA contractor Edward Snowden. You’ll probably have to get your recipient to download it, though.