What actually improves email deliverability in 2026: tools, content, or sender reputation?

I’m trying to understand what makes the biggest real-world difference in inbox placement today.

From your experience, what has helped most:

• Better list hygiene and authentication (SPF/DKIM/DMARC),
• Content and engagement optimization, or
• Switching to a different email marketing tool or setup?

Looking for practical insights and lessons learned rather than promotions. Thanks!