Previously when you sent an email, encrypted using the Office 365 Message Encryption, recipient would need to sign in using their Microsoft Account (or if they didn't have one, they would need to create one) to be able to view the message. Although this was (and is) a good solution for some customers, majority of customers didn't find it very convenient as it would not provide a great experience to the recipients who could be their business partners, suppliers or customers.
Now, there is a second option available to recipients who don’t have Microsoft Accounts (or don’t want to have one) and don’t want to sign in to view the encrypted messages. You can now choose the option “DON’T WANT TO CREATE A MICROSOFT ACCOUNT? GET A ONE-TIME PASSCODE TO VIEW THE MESSAGE” to access the encrypted message using a one-time passcode. You still will have the option to sign in using your Microsoft Account or if you don’t have one, you have an option to create one.
When you select the one-time passcode option, you will receive an email which contains a passcode, and a notification will be shown on the webpage that indicates a passcode is sent to your email address. You then need to get the passcode from your email and enter it in the webpage and click “Continue”. You will now be able to see the message in an OWA style webpage.
Read more: http://technet.microsoft.com/en-us/library/use-a-one-time-passcode-to-view-an-encrypted-message.aspx