Email forwarding is a service that lets you direct an email from your custom domain to an existing email account.
Email hosting isn’t provided on Shopify. Meaning that if you reply to the email then your personal email will appear, and not the email created through email forwarding. If you want to send an email from your custom domain forwarding address, then you need to use a third-party email hosting service. If you use Zoho Mail or Google Workspace, then you can connect the hosting service to your domain in Shopify. If you use another email hosting service, then you can add an MX record to connect it to your domain by editing your DNS settings.