nmail is described as 'light-weight console/terminal-based email client for Linux and macOS with a user interface similar to alpine / pine' and is an app in the Office & Productivity category. There are four alternatives to nmail for Linux, Mac, Windows and Xfce. The best alternative is Mutt, which is both free and Open Source. Other great apps like nmail are aerc (Free, Open Source), Alpine (Free, Open Source) and sup (Free, Open Source).
Alpine is a fast, easy to use email client for the command line that is suitable for both the inexperienced email user as well as for the most demanding of power users. Alpine is based on the Pine® Message System (http://www.washington.
Sup is a console-based email client for people with a lot of email. It supports tagging, very fast full-text search, automatic contact- list management, custom code insertion via a hook system, and more.