Earlier this year, Microsoft announced Office 365 Message Encryption service that allowed users to send encrypted mail to anyone. Today, they are announcing the release of One-Time Passcode. With One-Time Passcode, users need not to sign in with a Microsoft account to view an encrypted message.
Previously, when you received a message encrypted with Office 365 Message Encryption, you could view the encrypted message only by using a Microsoft account. Now, with the addition of the One-Time Passcode capability, you can view an encrypted message you receive without having to sign in with a Microsoft account. If you have a Microsoft account, you now have the option to view an encrypted message by selecting one-time passcode instead of signing in with your account.
This capability is now available to everyone in the respective tenants.
Read more about it here.