The primary criteria for segmenting an email list include demographics. Age is one important factor. Gender is another. You can also segment by location. Behavior is a key factor too. You can group people based on their actions like purchases or email opens. Interests play a big role in segmentation. You can segment by what your subscribers care about. Engagement levels matter as well. You should track how often someone interacts with your emails. By segmenting based on these criteria, you will send more relevant emails. Relevant emails lead to better results. These segments help you increase conversions.