When you first receive an email that has been encrypted the message will look like this:
You've received an encrypted message from email@example.com
To view your message
Save and open the attachment (message.html), and follow the instructions.
Sign in using the following email address: firstname.lastname@example.org
This email message and its attachments are for the sole use of the intended recipient or recipients and may contain confidential information. If you have received this email in error, please notify the sender and delete this message.
Open the email attachment which is an HTML file with instructions in it.
After downloading the Message you may receive a screen like this. At this point you can choose either of the options.
If you choose to sign in, you will be asked for the password to your email address.
If you choose the one-time password option an email will be sent to your email account with a one-time password.
Once you have received the email with the one-time password, copy the passcode into the window. If this was successful, you should be asked to reopen the attachment at which point you should be able to see the original email.