Hey ​@Paul808, thanks for a great question!

I’d say the right/good tool depends on which aspect of your deliverability you’re looking at.

Klaviyo offers a lot of tools to help you fine-tune your deliverability before you look outwards. You can now run rendering tests directly within Klaviyo (to check how your emails will render in different inboxes), and you can even check your email size and other elements directly within the UI.

Once you’ve confirmed these are all good, the next step would be to run some sort of a spam test, where our preference is GLOCK spam testing. These tests will give you a rough idea of inbox placement and your emails’ spam score, which will help you to improve things further.

Then, check your technical setup by using mxtoolbox, an older but still perfectly reliable tool for checking if your back-end email settings are all up to code and there are no loose ends in terms of IP health, DKIM, DMARC and a bunch of other things.

For monitoring, there’s nothing better than Google Postmaster and Outlook SNDS (those are the two biggest inboxes), together with reaching out to icloudadmin@apple.com if you notice any issues with your Apple deliverability (e.g. high bounce rate on Apple).

Let me know if this helped you find what you needed!