Should I Hire an Email Agency for Newsletter Subscribers?

Do you ever buy services from email list services? I have been running Meta ads to build the subscriber rate for newsletters, but I have given up as the CPA is all over the place. I have been thinking about hiring an email agency that specialises in finding the ideal subscribers for the newsletter This is kinda like cheating..... but want to know your thoughts on whether its a good idea or not? FWIW, I have run a few tests with them and the subscribers have a lower open rate and poor CTR..... but the agency said they will improve over time