Re: I've seen a definite uptick in these
... and to boot, it was not even one of the throwaway $(servicename)_$(qualifier)@$(mydomain) ones I expected.
In my case, $servicename == monsterboard. Nothing else.
Hmm, do they still exist? Apparently, yes. So I try logging in: nope. Password reminder? "No account associated with that mail address" or something like that.
And harvesting my contacts from there (insofar as they can actually do so, given a non-existent account) would yield a dozen or so spammy recruiters I haven't had any contact with for the past fifteen years, so I doubt they still work where they did back then.