(What can you do from a control perspective that is most likely to mitigate the...

Implement a method of data transfer for the files containing sensitive personal information with end-to-end encryption.

Specify in the customer contract that only an authorized end user is allowed to open files.

Allow employees to use their personal email to send files exclusively under emergency circumstances.

Keep a secure audit log of files with sensitive personal data sent to the customer including the intended recipient, and audit on a quarterly basis.