Subject line not populated

Hi,

I have a combination field [SumSubject] which adds a text field "Delivery" together with a field containing a value [XNumber]. Then I add the [SumSubject] to the subject line.

So far so good. It populates as I want to; for example: "Delivery 34".
But when I send this form the receiver it shows up in his inbox as "Delivery [XNumber]" not showing the actual number.

What I am doing wrong?

Thanks in advance
Smalling