Found this article quite useful!
While Gmail does not offer up a contact path to discuss deliverability issues directly with a Google representative, there is a way for clients to reach out to Google to ask them to reconsider spam folder delivery.
- Go to the Bulk Sender Contact Form and follow these steps:Fill out the form with your contact info and select the checkbox for “Your messages are incorrectly classified as Spam or Phishing.”
- Click the “next” button.
- Fill out the Issue Summary and Issue Description fields.
- Paste a full email message, with full headers, into the next field as directed. The email message must be one that was sent to a Gmail or Google Apps account. It must not be older than a few days old. It must include full email headers.