mailcontroller.cs

public class MailController : ApiController
{
    private readonly dynamic[] mail =
        {
            new { Id = 1, Location = "Inbox", Subject = "value1" },
            new { Id = 2, Location = "Inbox", Subject = "value2" },
            new { Id = 3, Location = "Inbox", Subject = "value3" },
            new { Id = 4, Location = "Deleted", Subject = "value4" },
            new { Id = 5, Location = "Sent", Subject = "value5" }
        };

    public IEnumerable<object> Get()
    {
        return mail;
    }

    public IEnumerable<object> Get(string location)
    {
        return mail.Where(o => o.Location == location);
    }

    public object GetById(int id)
    {
        return mail.FirstOrDefault(o => o.Id == id);
    }
}

routes.cs

config.Formatters.Remove(config.Formatters.XmlFormatter);

config.Routes.MapHttpRoute(
    name: "Mail by Location",
    routeTemplate: "api/mail/{location}",
    defaults: new { controller = "Mail" },
    constraints: new { location = @"[a-zA-Z]+" }
);

config.Routes.MapHttpRoute(
    name: "Mail",
    routeTemplate: "api/mail/{id}",
    defaults: new { controller = "Mail", id = RouteParameter.Optional }
);

routes.txt

/api/mail
/api/mail/3
/api/mail/inbox
/api/mail/sent