Solo isn’t an email host so Google Gmail (which costs money) or Zoho (free) are probably the easiest options to set up a custom email address for your custom domain.

To do so, navigate to Google Workspace and sign-up or visit this Zoho article to start. You will be asked to provide the name of your custom domain and then some instructions to configure your custom domain. Upon completion, you will now have an email address associated with your custom domain.