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Delivery is performed by SIP over TLS protocol, so as you can see the whole end-to-end path is encrypted and the message is stored after a local encryption on the server.
Send Secure Messages
It's possible to write Secure Messages to other parties both by answering them or by writing for your own initiative. In the first case you would probably use the Contact's History view, in the latter you would use the Contact's details view instead.
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