The point I was making is that whether or not you can successfully deliver email is not a sensible test of the validity of an email address, looking at the address purely as data. As I pointed out, my email archive contains many email addresses that are no longer ‘valid’ by your definition, but they are still valid as data.
By your definition email address validity changes literally on a moment to moment basis. Addresses are becoming invalid constantly and new ones are becoming valid constantly. It’s not a useful definition of validity, and not even something you can test meaningfully.
I've got your point already before and I think it's valid.
That's why I've formulated my "definition" carefully:
> the validity of an email address when someone shows you one
It's of course not a "definition" someone could write down into a spec. But It's by far the best "informal validity check" in practice. It checks whether an email address is currently valid. You practically can't do more anyway!
The "formal validity" of an email address changes with time nowadays as I've pointed out: It depends directly on the formal validity of the host name part which can change over time given the fact that the list of TLDs changes over time (which wasn't the case at the time those specs have been written; fun fact: there is more than one spec, and they're contradicting each other).
To add on that there are two more important aspects: Firstly an email address you can't send mail to is mostly worthless in practice as it can't be used for its primary purpose. Secondly even perfectly "valid" addresses (by the spec) aren't accepted by a lot of parties that claim to handle email addresses! I guess a lot of systems would for example refuse an address looking like "-@-", wouldn't they? But it's perfectly valid!
My initial argument was that claiming that it's "easy" to validated email addresses is wrong in multiple dimensions. In fact it's one of the more complicated questions out there (given the tragedy of the specs).
By your definition email address validity changes literally on a moment to moment basis. Addresses are becoming invalid constantly and new ones are becoming valid constantly. It’s not a useful definition of validity, and not even something you can test meaningfully.