If you are a Microsoft 365 subscriber, here is what is new to you: In an email message, choose Options, select Encrypt and pick the encryption that has the restrictions you want to enforce, such as Encrypt-Only or Do Not Forward.
Note: Microsoft 365 Message Encryption is part of the Office 365 Enterprise E3 license. Additionally, the Encrypt-Only feature (the option under the Encrypt button) is only enabled for subscribers (Microsoft 365 Apps for enterprise users) that also use Exchange Online.
For Outlook 2019 and 2016, In an email message, select Options > Permissions and pick the encryption option that has the restrictions you’d like to enforce, such as Do Not Forward.
Encrypt a single message
In the message that you are composing, click File > Properties.
Click Security Settings, and then select the Encrypt message contents and attachments check box.
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