Most "deliverability software" is a marketing page with a DNS lookup behind it. The difference between a real tool and a marketing page is whether it actually reads the signals the receiving mailbox uses, and whether it keeps reading them over time. Here is what to look for, and what is worth paying for.
A useful tool shows you the actual SPF, DKIM, DMARC, and MX records, not just a number. A score without the underlying records is a black box: when something breaks, you need to see which record is the problem. Look for a tool that shows the exact TXT value, flags the specific issue (a missing -all, a weak DKIM key, an SPF lookup over the limit), and tells you the fix. The DMARC checker and SPF checker do exactly this: they show the record and point at the problem.
Authentication is the floor, but reputation is what decides the inbox. A real tool checks the sending IP and the domain against the major blocklists and shows which lists you are on, plus the delisting path. If a tool only checks DNS records and ignores blocklists, it will tell you "you are fine" the day before your IP gets listed. The IP blacklist checker and blocklist checker cover this, and the Spamhaus delisting guide covers the hardest case.
Modern mailboxes verify the TLS certificate on your mail server. An expired or misconfigured certificate is a quiet deliverability killer that most "checkers" never look at. A serious tool verifies the certificate and flags anything that would make a receiver distrust the connection. The TLS checker does this.
This is the line between a free one-off check and software worth paying for. A one-time audit tells you the state today. But records break, IPs get listed, mailboxes get compromised, and the next campaign dies quietly. The value of paid deliverability software is continuous monitoring: check the domain daily, get alerted the moment a record changes or a blocklist hit appears, and fix it in minutes instead of discovering it from a drop in open rates. That is the core of Inboxproof Pro, and the Agency tier does the same for up to ten client domains with white-label reports you can hand over.
Be honest about what you are buying. A free one-off check is enough to find and fix the obvious problems: run the free deliverability checker on your sending domain and it scores all seven checks and points at the first thing to fix. You only need to pay when you need the state to be maintained: daily monitoring, alerts, multiple domains, an API, and white-label reports. If a tool charges for a one-time check, that is a bad sign. If it charges for continuous monitoring and multi-domain management, that is the part that is actually worth it.
Look for three things: it shows the actual records (not just a score), it checks reputation and TLS (not just DNS), and it keeps checking over time (not just once). Start free to find and fix the problems, then pay only for the monitoring that keeps you out of the spam folder. That is the whole job, and it is the whole reason a deliverability tool earns its price.