Kevin,
I found that Yahoo has an old legacy blocking filter. Somehow an
address got onto that list and then all messages from said address
were being accepted (thus no bounce) but not being received and not
appearing in either the spam or trash folders. They just simply
disappeared into the ether until the address was removed from
blocking filter!
That could be a problem with other email services as well. Some have a "block" or "blacklist" feature that is separate from either the spam filtering or the normal user filters.
When you say "legacy" do you mean that you had to find it in an unusual place in the user interface, somewhere a user wouldn't ordinarily look for filters or spam controls? My ISP email has one like that, a block list that predates the rollout of their current webmail interface (and a content-based spam filter). You have to go to a legacy configuration page to see it, it isn't in the webmail options page.
Shal
Shal Farley