https://www.xomedia.io/a-deep-dive-into-email-deliverability/

Sender Authentication: Senders should implement email authentication protocols like SPF, DKIM, and DMARC to prevent email spoofing and phishing attempts. We'll cover this in more detail later in this article.

Bulk Senders Requirements: Sending unsolicited bulk emails can lead to deliverability issues (spam filtering) and reputation damage. Email providers have various algorithms and user reports to identify and filter spam. Google will require bulk senders (those sending 5,000+ emails per day to Gmail) to meet stricter requirements for compliance with spam thresholds.

Easy Unsubscribe: Implement easy unsubscribe options (One-click Unsubscribe). Gmail users have tools to report spam, unsubscribe from unwanted emails and control their inbox experience. If it is too difficult to unsubscribe from your emails, customers will be more likely to flag your email as spam. Additional links provided in the 'References' section at the end of this article.

Engagement: Avoid misleading subject lines, excessive personalization, or promotional content that triggers spam filters. Focus on providing relevant and valuable information when considering email content.