"Validator" and "verifier" get thrown around as synonyms, but in deliverability work they describe two different jobs. A validator checks whether an address is well-formed - correct syntax, a plausible domain, no illegal characters. A verifier goes further and asks whether the mailbox actually exists and can receive mail.
Understanding the split saves you money and reputation. Validation is instant, free, and offline; verification requires network calls to DNS and mail servers, so it's slower and usually metered. Most teams validate first to strip obvious junk, then verify what survives.
