I am always delighted when #autocrypt just works with a new email contact.
#autocrypt is opportunistic email encryption: you send your public key within the header of every email. Then, your contacts can encrypt their replies to this public key. Here, opportunistic has a positive connotation: the protocol uses the best protection available, but does not guarantee a baseline better than plaintext.
It's available in Thunderbird, K9, and others. Read more about it on https://autocrypt.org/
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