How To Setup Up Email Sending Authentcation with my server?

I want my server to authenticate all outgoing emails from my server. At the domain provider, I record the SPF record and the authentication works perfectly. However, I also want my server to authenticate outbound mail from even a gmail email address. I sent the mail with PHPMailer and gave it a try, but when I read the mail, gmail prompts me that the email may not have been sent by the sender. The email header is not encrypted. For other email providers, I see that their own server authenticates even if they use gmail email addresses. What do I need to do to have my server authenticate outgoing mail from my server (including using a gmail email address). I want to build a newsletter sending service. Of course, I do not want to commit fraud and unethical activities. Customers can only use specific email addresses after they have previously activated it. So the email will only be used by the rightful owner of the email address. I just don’t know how to get my server to authenticate when sending these emails