MailCal uses EWS (Exchange Web Services) to connect to Exchange server. EWS allows the app to connect periodically in the background, and save and sync data on the phone. Usually when an organization opens up webmail access for checking mail via browser, EWS is also available.
But sometimes, a working webmail/Browser setup may not be accompanied with EWS access. It could be either because the admins have not fully configured it (inadvertently), or just might have explicitly disabled it.
In such cases, MailCal will not work for your account, even though some other apps, like (OWA Mobile Access for Android) might work. It is because those other apps use a browser widget to simply load the webmail content and reformat and display it within the app. But this is precisely such apps are limited in functionality (no period sync, no offline access, and sub optimal interface).
If you are not sure whether your organization supports EWS access, you can simply try to configure MailCal to find out. If account setup doesn't work, please use the options on error screen to send the error report to us and we'll advise you on the next steps.
In any case, you can always request a refund for your MailCal purchase - for any reason - at any time, so there's no cost to try.