Improving email deliverability for a B2C onboarding flow with many Gmail addresses involves a multifaceted approach encompassing technical setup, list management, content optimization, and continuous monitoring. Key strategies include implementing proper email authentication (SPF, DKIM, DMARC), maintaining proactive list hygiene, gradually warming up IP addresses for large-volume sending, and adhering to Gmail's bulk sender guidelines. Additionally, monitoring blocklist status, managing sending rates to avoid throttling, using preview tools for testing, avoiding spam trigger words, setting up feedback loops, segmenting your audience, and understanding your sender reputation with Microsoft are crucial for optimizing deliverability and ensuring emails reach the intended recipients.