The best cold email first line isn't clever, it's true. And nothing is more true than a complaint the prospect's own customer wrote in public.
Mining the reviews
Scrape recent 1- and 2-star reviews from the App Store, Google reviews, or G2, then extract the specific gripe: crashes on login, slow support, a missing feature. That sentence is your opener.
The angle
"Saw a recent review mention [specific issue], what's in place to catch things like that before they hit your rating?" It signals you did homework and it leads straight into the value you provide.
For a wellness-app client, referencing real 1-star reviews as the pain point dramatically lifted replies, because every message was undeniably about them.