Hi there,
Greeting for the day, hope you are doing well
I have 10+ Years of experience as Hosting Support Engineer.
as per your description you wanted to use third party SMTP server to send Oracle Apex confirmation email . here I will recommend you to use sendgrid SMTP server for email confirmation relay . since if we choose to use google SMTP it has also limitation if more than 2,000 email sent a day you will start experiencing rejection from google. so best option to use professional SMTP provider which built for such requirement . I will help you in setup of sendgrid SMTP for you .
Please initiate chat to discuss your requirement in details.
Thanks
Karun