Way to secure outgoing e-mail for +1500 diffrent exchange users

GDPR new rule needs to password protect send emails containing user data (name, phone, family, etc...) but there is no real option in outlook to password protect an e-mail. does anyone know how this can be done? we have around 1500 user lailboxes. So installing an S/MIME certificate for each users does not seem like a good thing. Installing over 1500 certificates is a big and expensive job. ideas suggestions??