Here is a simple way to encrypt an email in Outlook that can be decrypted using any mail program and an explanation, from Microsoft, of what the options mean.
Do Not Forward: Your message stays encrypted within Microsoft 365 and can't be copied or forwarded. Microsoft Office attachments such as Word, Excel or PowerPoint files remain encrypted even after they're downloaded. Other attachments, such as PDF files or image files can be downloaded without encryption.
Encrypt: Your message stays encrypted and doesn't leave Microsoft 365. Recipients with Outlook.com and Microsoft 365 accounts can download attachments without encryption from Outlook.com or the Outlook mobile app. If you're using a different email client or other email accounts, you can use a temporary passcode to download the attachments from the Microsoft 365 Message Encryption portal.