PlusVibe

PlusVibe (it used to be Pipl.ai) is a good fit if you want to resell cold email to clients under your own brand. The Agency plan sets up a separate backend for each client with white-labeling and unlimited users, starting at $497/mo for 500,000+ emails. Smaller teams can start at $37/mo for 25,000 emails a month, and there's a 14-day free trial.