Setting up Craft's registration e-mails to be sent through GMail
I'm not sure how your email address got blocked, but Craft 3 uses the same SMTP smtp.gmail.com server and settings that Craft 2 uses when you choose to send email via "Gmail"

https://github.com/craftcms/cms/blob/develop/src/mail/transportadapters/Gmail.php#L91-L97

Sending email through PHPMail/sendmail is notoriously unreliable and I wouldn't recommend it on production.

https://craftcms.com/support/troubleshooting-email-errors#dealing-with-email-delivery-issues

And there are quite a few steps you can take to help ensure your email doesn't get treated as SPAM: https://blog.codinghorror.com/so-youd-like-to-send-some-email-through-code/