Emails from email form goes to spam folder

Hi I have an email form in my website which is working fine. It sends the mail to my Gmail however, Gmail marks those mails as spam.

It's an email form with an anti spam system found here.

My question is: how can I prevent mails sent from my contact form (or my future contact forms that are yet to be made) from being marked as spam by gmail or other email clients?

PS: I am writing this on mobile and I cannot upload the code yet. As soon as I hit the office I'll try to edit this and append the code.