I got the MAPI branch compiled from source (revision up-to-date as of today). I could access the server and see mail (but not read them because character encoding was screwed up - a different problem).

Accessing calendars didn't work. It asked for a password each time I tried to activate a calendar and reported an authentication error in the console log.

I traced it down to e_cal_backend_mapi_authenticate() in e-cal-backend-mapi.c which called exchange_mapi_connection_new() a) without the necessary password and b) with the wrong profile string.

Adding a call as it is done camel-mapi-store.c solved the problem. For the sake of illustration I left the old call in (also as fallback), but I seriously wonder how that code could have possibly worked.

Here's my change:

Index: calendar/backends/mapi/e-cal-backend-mapi.c
===================================================================
--- calendar/backends/mapi/e-cal-backend-mapi.c (revision 9450)
+++ calendar/backends/mapi/e-cal-backend-mapi.c (working copy)
@@ -104,7 +104,11 @@
        cbmapi = E_CAL_BACKEND_MAPI (backend);
        priv = cbmapi->priv;
 
-       if (authenticated || exchange_mapi_connection_exists () || exchange_mapi_connection_new (priv->user_email, NULL)) {
+       if (authenticated || exchange_mapi_connection_exists () ||
+           /* this is how the function is called in camel-mapi-store.c, which works by using the default profile */
+           exchange_mapi_connection_new (NULL, priv->password) ||
+           /* this call doesn't pass the password and uses the wrong profile (first.last@mail.domain instead of account@windowsdomain) */
+           exchange_mapi_connection_new (priv->user_email, NULL)) {
                authenticated = TRUE;
                return GNOME_Evolution_Calendar_Success;
        } else {