Your segment is fine, your volume is wrong in the other direction. 400 emails to a scraped directory is 400 strangers. Ten emails to ten clinics where you know the owner's name, what software they run and one specific thing about their setup will beat it.
I sent 34 emails total to get my first six customers. Each took about 12 minutes because I actually opened their site first. That's roughly seven hours of work for six customers. Sending 400 templated ones is also hours of work and you have zero.