How to setup SMTP authentication for a backend-frontend contact form in a Linux environment (CentOS)?

On a CentOS-Bash shared hosting environment I host a website with a simple backend-frontend contact form with the PHP mail() function. Test emails can reach my Gmail account but if they do reach it at all (which is very rare), it takes much time, usually more than an hour. --- I understood from an infrastructure developer from my hosting company that this is because: * The emails are eventually sent from RANDOMLY_CREATED_USER_@us153.siteground.us which doesn't have valid DKIM, valid SPF, and SMTP authentication The infrastructure developer suggested that to solve this problem I would open an email account in my hosting environment and refer emails to there, so then I would indeed have valid DKIM, valid SPF and SMTP authentication. --- But say that I still want to refer emails to my Gmail account;
How to setup SMTP authentication for a backend-frontend contact form in a Linux environment (CentOS)?
Would that even be possible on a shared hosting environment (and then what about DKIM and SPF?)... ## Notes * Another problem added upon the one described above occurred with a second form which is a bit more complex than the simple test form in the base of this session;
The "From" => $email *header's email address* was my own Gmail address so an eventual sending from RANDOMLY_CREATED_USER_@us153.siteground.us was mistakenly grasped as *email faking* ("spoofing"), again, on top of the server mismatch described above