Forwarding Service helps you to forward your custom domain email into your mailbox and using Forwarding Service you would not be able to send emails. But here is the workaround for that: https://calderaforms.com/2019/05/smtp-gmail/
This guide will help you to whitelist your entire domain name: https://www.marottaonmoney.com/how-to-whitelist-an-entire-domain-in-gmail/
* In the "To" box type your domain name instead of the "From" box.
Absolutely yes, we also protect yourselves from spam and malicious emails.