A few months ago I ran a campaign to ~14,000 emails I'd collected through legitimate signups. 2,100 bounced. Our sender reputation tanked, mailbox providers throttled the next three sends, and the

open-rate cohort took a quarter to recover.

The infuriating part: most of those addresses looked fine. Valid format. MX records present. A free verifier had marked them deliverable.

Turns out most email verifiers stop at the easy checks. Format and MX get you ~60% of the way. The remaining 40% disposable inboxes (Mailinator etc.), role addresses, typo'd domains (gmial.com),