When a domain or IP address experiences a spamming incident, resulting in a lowered reputation on Google.
To prevent mail to Gmail users from being blocked or sent to spam, fill out the form given below, and Google will whitelist your domain.
Fill in all the details like Name, Email address and Please select the option "Your messages are incorrectly classified as Spam or Phishing", then click Next.
Then fill in the short summary and detailed description of your issue, then Paste the email header in the given box. Then submit.
The form link is as follows: