The actions to take in order to open a protected (encrypted) email message received from the EUI differs based on the email account (GMail, Hotmail, Yahoo, etc.) or email client (Outlook 2016, Outlook 365, Outlook Webmail, etc.) used. As a general rule:

• If you use Outlook 2016, Outlook 365 and Outlook Webmail: you should be able to open the message like any other message. You will see a banner at the top of the message which informs you of any restrictions on the message.
• If you are using a different email account (GMail, Hotmail, Yahoo, etc.) or email program (Apple Mail, etc.), you may receive a notification that you have received a protected message and need to take additional action.

You may find detailed step-by-step instructions at the following Microsoft page:

How do I open a protected message?