Inquire if your clients use Wickr, an instant messaging app often used in business to send secure messages. Wickr employs end-to-end encryption so that messages are encrypted even when they are stored. It includes a 'seff-destruct' option which will automatically erase messages shortly after they are sent, so it can function like SnapChat. Wickr is a high end product - it can be implemented for 100 users at a cost of $50,000. It's a good alternative to collaboration tools like Slack that lack an encryption feature.
Wickr's web site notes an important exception to the requirement of the GDPR that data security breaches be reported immediately. Notice is not required under Article 33 if encryption has rendered data unintelligible to unauthorized users. Another good reason to send business related messages with encryption.