A message that is encrypted by Office 365 Message Encryption is delivered to a recipient’s inbox just like any other email message. If the recipient has Outlook 2013 or 2016 and an Office 365 email account, they'll see an alert about the item's restricted permissions in the Reading pane. After opening the message, the recipient can view the message just like any other.
Note: We recently released the encrypt-only policy in Outlook on the web. Today, messages that have the new encrypt-only policy applied can only be read directly in Outlook on the web and in Outlook for iOS and Android. We plan to release the ability to send, view, and reply to messages with the encrypt-only policy in other Outlook clients in the near future. In the meantime, the ability to send, view, and reply to messages with the encrypt-only policy for Outlook for PC is available only for members of the Office Insider program. Other customers will see a message with a link. That link will take Office 365 users to Outlook on the web to read the message. Users with other email accounts will be prompted to obtain a one-time passcode and read the message in a browser window.
If the recipient is using another email client or email account, such as Gmail or Yahoo, they'll see a link that lets them either sign in to read the email message or request a one-time passcode to view the message in a web browser.
Send an encrypted message using Outlook for PC
There are two primary ways to send encrypted messages.
Your admin can define rules to automatically encrypt messages that meet certain criteria. For instance, your admin can create a rule that encrypts all messages sent outside your organization or all messages that mention specific words or phrases. Any encryption rules will be applied automatically.
If you want to encrypt a message that doesn't meet a pre-defined rule or your admin hasn't set up any rules, you can apply a variety of different encryption rules before you send the message. To send an encrypted message from Outlook 2013 or 2016, or Outlook 2016 for Mac, select Options > Permissions, then select the protection option you need. You can also send an encrypted message by selecting the Protect button in Outlook on the web.
View and reply to an encrypted message for Office 365 recipients using Outlook for PC
If you're in an Office 365 organization, you can read messages encrypted with the do-not-forward policy or custom protectiontemplates in Outlook 2013 and Outlook 2016 for PC, Outlook 2016 for Mac, Outlook on the web, Outlook for iOS, and Outlook for Android. Currently, you can only read messages set with the encrypt-only policy directly in Outlook on the web, Outlook for iOS, and Outlook for Android unless you are a member of the Office Insider program. If you're using Outlook for PC and are not a member of the Office Insider program, click the link included with the message. Office 365 users will be taken to Outlook on the web to read the message. Users with other email accounts will be prompted to obtain a one-time passcode and read the message in a browser window.
To reply to an encrypted message
Choose Reply or Reply All.
On the page that appears, type a reply and choose Send. An encrypted copy of your reply message is sent to you.
View and reply to an encrypted message without Office 365 using Outlook for PC
If you're not using Outlook with Office 365, your encrypted message will contain a link in the message body.
Select Read the message.
Select how you'd like to sign in to read the message. If your email provider is Google, Yahoo, or Microsoft, you can select Sign in with Google, Yahoo, or Microsoft respectively. Otherwise, select sign in with a one-time passcode.
Once you receive the passcode in an email message, make a note of the passcode, then return to the web page where you requested the passcode and enter the passcode, and select CONTINUE.
Tip: Each passcode expires after 15 minutes. If that happens, or if you can’t open the message for any reason, start over by opening the attachment again and following the steps.