A clean list does not guarantee inbox placement. Filtering decisions are made on the sender, not the recipient, and the four most common causes are:
Authentication
SPF, DKIM or DMARC missing or misaligned, which Gmail and Yahoo now treat as a bulk-sender requirement rather than a recommendation. DMARC monitoring shows what is failing and why.
Reputation
The sending domain or IP has a history filters do not like, or is listed somewhere. Blacklist monitoring catches a listing before a campaign does, and an email health check covers the rest of the sender profile.
Content and structure
Image-to-text ratio, link domains that do not match the sending domain, or a subject line that pattern-matches promotional mail. A content spam check scores a message before it goes out.
Engagement
Low opens and clicks from the recipients a provider has already seen, which pushes later messages to Promotions regardless of content. Email list validation removes the addresses that never engage because they are not real.