axios({
      url: 'https://api:key-someapikey@api.mailgun.net/v3/somesandboxdomain1c.mailgun.org/messages',
      method: 'post',
      username: 'api',
      password: 'key-somekey',
      data: {
        from: "Excited User <mailgun@some.example.org>",
        to: "myemail@gmail.com",
        subject: "Hello from react app",
        text: "Testing some Mailgun awesomness!"
      },
      headers: {
        "Content-Type": "application/x-www-form-urlencoded",
        'Authorization': `Basic ${window.btoa('api:key-someapikey')}`
      }
    })
      .then(function (response) {
        console.log(response);
      })
      .catch(function (error) {
        console.log(error);
      });

XMLHttpRequest cannot load https://api.mailgun.net/v3/somesandbox.mailgun.org. Request header field Authorization is not allowed by Access-Control-Allow-Headers in preflight response.

axios({
    method: 'post',
    url: `${mailgun.baseUrl}/${mailgun.domain}/messages`,
    auth: {
        username: 'api',
        password: mailgun.apiKey
    },
    params: {
        from: 'Awesome Development Team <noreply@yourdomain.com>',
        to: 'testing@example.com',
        subject: 'Hello',
        text: 'Welcome to the team!'
    }
}).then(
    response => {
        console.log(response)
    },
    reject => {
        console.log(reject)
    }
)