I received an email this morning for a $10 Microsoft Store gift card from the Microsoft Store. The email title is "Only 1 day left: Your free $10 gift card is about to expire" and the original email was "Here's a $10 gift card just for you" on the 17th for me. Not sure how widespread this is, but figured I would post a heads up.
Here are the terms:
Valid in US Only. Promotional gift card. Must redeem by November 30, 2017 and spend within 90 days of redemption. Works with Microsoft Store online, Windows 10, Xbox One and Xbox 360 (Xbox Live required). Does not work at physical Microsoft Stores. Once redeemed to your U.S. Microsoft account, the full code value will be applied and may be used for eligible purchases (exclusions apply) made directly at select Microsoft digital stores. Eligible purchases and prices vary by region, device, and over time. Geography limitations, country and balance restrictions, taxes, and Internet connection fees may apply. Paid subscriptions required for some content. Age restrictions apply. Except as required by law, credit cannot be redeemed or exchanged for cash and is not refundable. To read full terms and conditions (which may change without notice), go to microsoft.com/cardterms. Void where prohibited or restricted by law. Cards and codes issued by and ©/™/® Microsoft Corp, a Washington Corporation, and/or its affiliates.