the error occurs because, as it says, [Email] is part of a compound key and you need to filter on all columns of the compound key
what is important to mention here is the fact that also a name-column in multidimensional is reffered to as a compound key in DAXMD
when you run the following query:
you will realize that you get (at least) two columns for your original [Email]-attribute
one of them is suffixed as [Email.Key0] - this refers to what you defined as key-column of your multidimensional attribute. the other column is [Email] without any suffix.
When you use CALCULATETABLE it expects the columns and values in that column to be unique which is not necessarily the case if you only filter by [Email] as two persons may have the same email-address (theoretically)
so you need to filter on [Email.Key0] if you want to use CALCULATETABLE
an other option is to use FILTER instead of CALCULATETABLE which will work in both scenarios as FILTER does not rely on unique values in a column, though FILTER might be slower
hth,
gerhard