Received an abuse complaint from an ISP, what now?

First, don't panic — a single complaint is normal and expected at low rates (most senders see some baseline complaint percentage even with fully opted-in lists). What matters is the rate, not any individual complaint. Can you tell me roughly what your complaint rate has been trending, and whether this feels like an isolated incident or part of a pattern?

At roughly 0.15% that's within a generally acceptable range (most guidance puts the danger zone above 0.1–0.3% sustained), so you're not in immediate trouble, but it's worth watching. A few things that reliably reduce complaints: make sure your unsubscribe link is obvious and works instantly (frustration with a broken/hidden unsubscribe is a top complaint trigger), and double check you're not accidentally re-sending to people who already unsubscribed or asked to stop.