The early days of using email encryption often meant that your correspondents had to use the same email vendor that you did to pass messages back and forth securely. That is thankfully a thing of the past. Today’s encrypted products make use of “zero knowledge” systems, meaning that you can send an encrypted message to someone that isn’t yet setup on your chosen encryption service. In many cases, you don’t have to pay extra to send messages to these external users either, so everyone wins.
All you have to do is tell them (via some other communications method) a passphrase to decrypt their message and to compose a reply to you, or in some cases they can read the message by just authenticating themselves. After this first communication, your recipient is now able to exchange encrypted messages with you quite easily. Some email products, such as HP/Voltage, add a few additional steps as part of this process but still isn’t anywhere as onerous as it once was.