There's a discussion on the slickdeals version of this. From what I gather, the coupons are valid but poorly run.
One Qdoba manager from Kentucky claimed that the using the coupons (if you hadn't personally received it in an email) was equivalent to coupon fraud. However, someone pointed out that the coupon is not altered, and simply says limit one per a customer. The email may state an intended recipient but that name is not part of the coupon.
My view on the matter is that the coupon is legal, whatever Qdoba's intentions may have been. Whether the store accepts it seems to vary with location; some accept it, some may give you dirty looks, and some may refuse the coupon.
I think this would best be summed up as YMMV.