could be something like this :
CREATE TRIGGER TrIU_T_Mytable ON dbo.Mytable
FOR INSERT, UPDATE
AS
IF MyEmailColumn is NOT null
begin
-- must be unique where not null
If exists (select * from dbo.Mytable T inner join inserted Ins where T.MyEmailColumn = Ins.MyEmailColumn) begin
raiserror("Duplicate Emailaddress", 12,1)
rollback transaction
end
end
GO
test it test it test it