What are the best cost-effective services to get mailboxes and domains for Apollo at scale?

For affordable mailbox and domain setup at scale, consider these options: Zapmail with Instantly at $3/month per inbox (handles domain registration and DMARC setup, can export to Apollo), theboomerang.co at $2-2.25 per mailbox, or Warmy and Mailreach if you specifically need email warming for new sending domains. Zapmail is particularly convenient as it manages the technical setup including domain registration and DMARC configuration.