Return Path calculates Sender Score using a variety of metrics, including:
Complaint rates: The percentage of recipients who mark your emails as spam. Bounce rates: The percentage of emails that are returned as undeliverable. Spam trap hits: The frequency with which your emails hit spam traps, which are set up to catch spammers. Blacklist presence: Whether your sending IP is listed on any email blacklists. Sending volume: The number of emails you send can also affect your score, particularly if there are sudden spikes.