MailCal connects to Exchange Server using the EWS (Exchange Web Services) protocol. Most exchange servers that expose webmail also have EWS usually open and it is used by many MS-Office communication products.
MailCal connects directly to your Exchange Server, and unlike some other EWS apps on PlayStore uses NO intermediary servers. This means no compromise on security and privacy as your credentials and your data never traverse any third party servers.
Exchange Admins, if they track the activity, would see MailCal activity as generic EWS sync calls. Such calls are made by a number of Communication applications, including Desktop Outlook and Lync clients.