Please read the following ADS support document about email encryption:
https://vermontgov.sharepoint.com/sites/ADS-IT/HowTo/Forms/AllItems.aspx?viewid=0866c361-fc54-4f09-b317-213ea1eec2b3&id=/sites/ADS-IT/HowTo/Encryption%20--%20Microsoft%20Office%20365%20Instructions.pdf&parent=/sites/ADS-IT/HowTo