Email validation should be Pay-as-you-go compatible
There is Flex plan which aligns with modern approach to Pay-as-you-go but unfortunately it does not include Email validation feature.
Hence, the feature is not Pay-as-you-go compatible which feels as inconsistency in Mailgun offerings.
I'm pretty sure it's possible to increase per-validation pricing high enough to make having it in Flex commercially resonable.
From another side, I would like to have this option charged on Pay-as-you-go basis along with other features of the Flex plan.