On Your Office Outlook Web Email. Please see the image below:

Select "options" (...) to show a list of dropdowns.

Select "Encrypt" and then "Encrypt" (for most of the entities there should already an option for "Encrypt".

Click on "Encrypt" to make sure the current email is encrypted.

How to know if your existing email is already 'Encrypted'. Once 'encrypted', there is an message tip on top of the "From" shows the indication.

"Encrypt: This message is encrypted. Recipients can't remove encryption." 

Here you can "Change permissions" or "Remove encryption"

So whatever email content and file attachments in this 'encrypted' email will be sent via encryption.

Thank you.