Similar questions have come up in this channel a few times. I think it's not a good idea. I would recommend restricting Malli to verify that the data is of the right "shape", not that you haven't seen that email before. (Nor that it's valid, for that matter, since only really attempting to send a validation email can do that.) That said, if you have an entity with multiple email addresses in it, you can certainly check that all those are distinct from eachother with Malli.

seems reasonable but what's your rationale behind thinking it's a bad idea?

Performance cost is one reason, and difficulty of testing. https://clojurians.slack.com/archives/CLDK6MFMK/p1706115115762729?thread_ts=1706111434.068599&cid=CLDK6MFMK There's also another thread here that you might find interesting: https://clojurians.slack.com/archives/CLDK6MFMK/p1691330835332889

Doing side effects in the validation predicate sound like folly. I wouldn't do anything that needs that in it.

I wouldn't say that doing a read operation from the db counts as a side effect, but I agree that this type of validation may be best done outside of malli

It might be neat to opt into it based on context.