If you are the recipient of an encrypted email through Office 365, you will receive an email containing the following:

You've received an encrypted message from username@uwstout.edu
To view your message
Save and open the attachment (message.html), and follow the instructions.

Open the attachment After downloading and opening the attachment, either sign-in to view using an existing Microsoft account (UW-Stout or home) or by choosing the one-time passcode option.

The "Sign in" option requires a Stout username and password; select the "Work or school account" option.

If the "one-time passcode" option is selected (for any non-campus recipient), the recipient will receive a second email containing the passcode within a few moments of choosing that option.

Should the correspondence require a reply, as long as ‘encrypt email’ remains in the subject line, all subsequent emails will remain encrypted.

Any message sent through Office 365 that contains ‘encrypt email’ within the Subject line will be encrypted. Encryption level meets FIPS 420-2 Security Standard.

Reference
https://www.microsoft.com/en-us/TrustCenter/Compliance/FIPS