Quickest way to find out someone's email?

assuming this is for business outreach: check the company site first (about, team, press pages), then their LinkedIn. If you have a name and the domain, work out the company's pattern from any one known address (first@, first.last@) and run it through an email verifier before sending. Hunter's free plan covers a small number of lookups a month. Don't guess and blast, bounces hurt your sender reputation quickly.