Hi,
the values accepted by Outlook depend on Outlook version, runing standalone or with Ms Exchange, MS Exchange version etc..., actually this should be documented by Microsoft, but it is NOT. Sometimes it even depends on combination of values, so when Col1 value is X it is OK, but when Col4 value is Y the X is not OK anymore.
I know it is very frustrating, but same here! We have tryed to define some set per column, but we give up!