Just to confirm, is this device being enrolled under Android (Legacy) and not Android Enterprise? Assuming that's the case, most older Android devices did not support management of the native email client. Certain manufacturers certified certain devices with additional MDM capabilities (such as SAFE v1 for Sony, LG Enterprise v1 for LG, Sony Enterprise v4 for Sony devices); but this was only available for devices approved by the manufacturer, that also as the appropriate OEM service app installed. Other apps such as Gmail or Boxer (formerly AirWatch Inbox) tended to work much better in these scenarios.
Could you give some specifics of the exact device make and model you're testing with? If you go into the Device Details in WS1, do you see anything listed in "Enterprise Version" under the "Device Details" section?