Here's the legal citation, from the South Dakota Commercial Code:
47-1A-1420. Grounds for administrative dissolution. The Office of the Secretary of State may commence a proceeding, under §47-1A-1421, to administratively dissolve a corporation if:
(1) The corporation does not pay within sixty days after they are due any filing fees or penalties imposed by this chapter or other law;
(2) The corporation does not deliver its annual report to the Office of the Secretary of State within sixty days after it is due;
(3) The corporation is without a registered agent in this state for sixty days or more;
(4) The corporation does not notify the Office of the Secretary of State within sixty days that its registered agent has been changed or that its registered agent has resigned or
(5) The corporation's period of duration stated in its articles of incorporation expires.
And here's the letter I received: