Gmail and Yahoo have announced new sender requirements taking affect in February of 2024. You can read more here: https://help.convertkit.com/en/articles/8538056-gmail-and-yahoo-s-new-requirements-for-2024-and-what-to-expect

The 3 main steps to take:
- Set up a verified sending domain in your email provider
- Set up a DMARC record if you haven't already
- Start monitoring your spam complaint rates within Google Postmaster Tools