They provide a wide-ranging tool to view your email templates in many different email clients under different circumstances. We used this tool to identify specific issues we couldn't test that a small subset of users had complained about.
The Seedlist Optimizer Tool does the legwork for you to identify which ESPs are used the most and gives you a custom seed list for testing.
The Campaign Diagnostics report shows immediate feedback after sending on how your email is performing and where you can improve in the future.
Within the 250ok dashboard, there are a few different types of tracking and monitoring tools that aren't explained well within the product. Their uses were explained during onboarding but we continue to go back to the self-help to confirm best practices. I hope they work on more contextual help to improve this experience.
While the product does work if you have shared IP addresses, it took us some time after launch to filter out other's email data that get included during monitoring.
250ok, acquired by Validity in 2020, offers what is provided by the vendor as a complete email deliverability solution, including tools for deliverability analytics, design, DMARC, engagement, and support by email experts.