Your employees are constantly emailing out documents of a sensitive nature. This could be client information or financial data or even a new business pitch. However, anything sent via email is viewable by any mail server that handles the message along the way. Also, the sender has no control over who handles the message or how many copies of it are stored on mail systems along the way. Therefore, attaching such confidential files to an email that anyone can open / read is risky and irresponsible.
Your business needs to understand and mitigate these information security risks.
Ensure that when employees have to share sensitive documents, they use a file sharing service that requires the recipient to enter a password. Another alternative is to request employees not to send confidential attachments via emails that are not encrypted.
One more way of safeguarding your organization's sensitive documents is by providing employees with links to file sharing services (such as Dropbox or Google Drive) that they can use to protect attachments via a password. During onboarding / training, show employees how and when to use these services and send via encrypted email, explaining that this is company policy.