Yahoo are one of the original dotcoms. They've been around for a long time so they should know their business. Imagine my surprise when one of my clients starts complaining that their confirmation emails to yahoo email accounts are permanently being binned as is everything else they send - including personal communications.
Like most mail providers, free or otherwise, Yahoo have a spam policy that will look at an inbound email and then drop it in your inbox or spam folder depending on how it is classified.
As with most techies I have about a dozen email addresses at various providers in order to test exactly these sorts of issues. Especially given that the goalposts are changing all the time.
Sure enough even a personally addressed confirmation email was killed as it came into my yahoo account. "Ah ha," said I, "they've been blacklisted". So off one goes and checks the various blacklisting sites and there's nothing there. Hmmm.
It transpires that yahoo have just taken it on themselves to block that domain. Weirdly though, a personally addressed mail to me from the client with only the word "test" in the subject line is still considered Spam yet an email from some random address that doesn't reply, containing several instances each of the words "penis", "cock", "viagra" and "cialis" made it through to my inbox completely unscathed. At this point the phrase about arses and elbows definitely comes to mind.
Trying to get Yahoo to do anything about this issue is similarly problematic as there are no feedback channels to deal with this problem at all.
So overall we've just had to advise people to not use Yahoo or to check their junk mail periodically and read the mail there.
Update - The guys at Yahoo came to our rescue after tracing through the "network" somewhat to find someone that knows someone at Yahoo to help us out. Unfortunately their techies couldn't explain why we'd been black listed either but we are now officially on their whitelist so big thanks to the guys for helping us out.
Permanent source for attribution: https://www.ajfisher.me/2008/04/21/can-yahoo-really-get-things-so-wrong/.