What's the difference between domain reputation and IP reputation?

IP reputation is tied to the specific mail server IP address sending your email. Domain reputation is tied to your sending domain (the From: address). Both matter. If you use a shared email service (SendGrid, Mailgun, Mailchimp), you’re sharing IP reputation with other senders on that IP pool — which is why their IPs can sometimes carry neighborhood reputation problems. Your domain reputation is entirely yours. This is why high-volume senders on shared infrastructure eventually move to dedicated IPs — to own their IP reputation as well as their domain reputation.