I first noticed Yahoo's problem with EATING both incoming and outgoing email 9 years ago. At the time our software project lost touch with our lead developer for 6 months because somehow neither mail to nor from his Yahoo account went anywhere.
I whined about this on another forum, and someone there, savvy in the ways of email servers, replied that he had recently run a simple test query against all of Yahoo's mail servers, and thereby discovered that about a THIRD of them were drastically misconfigured -- and THAT is why they were randomly eating mail.
Yahoo Groups is subject to the same problem -- sometimes it works, sometimes a given group becomes an email black hole.
So... whether your Yahoo email or group works depends solely on the luck of being on one of the "good" servers.
As to spam, I suspect this is a similar issue -- that some servers are configured correctly, others are not, and the latter get spammed all to hell. As evidence, I offer this:
I've had a Yahoo account for 10 years, and for its first 6 years it never got a single spam. AFTER Yahoo introduced spam filtering, it began getting a trickle (one or two a month). I haven't actually logged in there in over a year (since I hardly ever use it for real email) ... so let's go see what I've got! [goes off, logs into Yahoo mail] In a year's time, my Yahoo mailbox has accumulated -- count them -- just 13 spams.
I don't think it's coincidental that on those rare occasions when I *do* use my Yahoo account, it does NOT seem to lose any email, especially since I've also noticed that those Yahoo groups most infested with spam are also the same ones that sometimes go down a black hole for several months at a stretch.
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~REZ~