Before increasing volume, make sure the basics are boring.
Identity and infrastructure
Verify SPF and DKIM on real messages, deploy DMARC, maintain valid DNS/rDNS for sending infrastructure where applicable, and use TLS.
Recipient operations
Know where addresses came from. Process permanent failures, complaints and unsubscribes. Keep promotional/subscription traffic operationally distinct from critical transactional messages where practical.
Message standards
Generate standards-compliant messages and implement provider-required unsubscribe behavior for applicable mail.
Monitor before scaling
Watch delivery responses, complaint signals and provider dashboards as volume changes. A large launch is a bad time to discover an old application is sending unauthenticated mail from the same domain.
Provider rules move
Google currently publishes specific requirements for higher-volume Gmail senders. Other mailbox providers maintain their own policies. Recheck them before major campaigns rather than relying on a years-old checklist.