I used “Input Prompt” for a situation like this, and I had no problems. This should work as Stuart suggested. Stewart is also right that this is not possible at this time.
You can also check Telerik controls, RadWindow for WP7 to be specific, should solve the problem well. The only thing is that they are commercial, so you will need to purchase a license.
But if you want to create your own from scratch, I watched the video “ Create a custom pop-up, ” on silverlight.net, which you can start well. It basically gives you a “pop-up”, and you can add any other controls that you need inside.
Keep in mind that all of these solutions can be styled and positioned to "mimic" the look of the mailbox as you like. Hope this helps.
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