Improve emails being marked as spam

https://support.google.com/mail/answer/81126?hl=en https://support.google.com/mail/troubleshooter/2696779?hl=en https://support.google.com/mail/troubleshooter/2920052?hl=en

• DKIM
• DMARC
• PTR
• Add SPF to domain provisionning script