...It helps developers test layouts, content, and delivery behavior without configuring a real mail server or risking accidental messages. In Rails, it works as an Action Mailer delivery method and stores generated messages locally. Developers can customize the storage directory, browser-opening behavior, message template, and file URI scheme. The gem also supports non-Rails setups using Mail, Pony, or Action Mailer directly. Because it relies on a local browser through Launchy, remote servers, containers, and virtual machines may require adjusted settings or alternative mail-preview tools.