Domain reputation is a crucial factor in email deliverability, as mailbox providers assess how your domain is perceived by receivers. It's calculated based on complex algorithms that evaluate the performance of your messages and their impact on inbox placement. Your domain doesn't have a single reputation; instead, it has multiple reputations unique to each receiver's proprietary scoring processes. To gain insight into your domain's overall health, you can use free online tools such as Google Postmaster Tools, Talos Intelligence, third-party filtering software like Barracuda and McAfee, SpamAssassin filters, and public block lists. By monitoring these tools and following best practices for improving your domain reputation, you can optimize deliverability to mailbox providers like Gmail, Yahoo, and others.