If you're doing something right, and it's making an impact, your competitors will pick up on it. If your competitors start changing their own strategies and tactics, and if it seems like they might be doing this in response to your campaign, it's worth looking into. Identify your main competitors and analyse how their own email messaging has changed (if at all) during your campaign.
That isn’t to say that sales-y promotions couldn’t ever create results—as marketing “gurus” have proved. The results just aren’t as good as they could be. Take the biggest gurus’ results—the ones they boast about—and calculate their conversion rates. Often their marketing is comparatively ineffective. They just have massive volume, so the sales numbers are impressive.