Can I continue to use my email service provider with a custom domain?
In technical terms, TurboSteed only takes control of your domain and the associated 'www' CNAME (i.e., yourdomain.com and www.yourdomain.com). This allows you to configure (or leave unchanged) other DNS records used by your email service provider in your DNS configuration.
Therefore, you can continue using your own email service provider even after configuring the custom domain for your site