Via the smartphone of your collaborators, anyone can get access to their e-mail correspondence. This access is often less protected than the PCs and desktops they are using.
Ensure that all the phones of your staff members go into locked mode automatically after some minutes of inactivity. It is pretty easy for everyone to do that by themselves.
Moreover, do incorporate the access to the phones in the code of conduct of your employees, just like you would mention the access to the PCs. That way, the devices are at least less accessible, and it will be more difficult for intruders to hack your mail systems.