Malling list fun!
I host a small collection of domains on my public web server, doing the usual run of the mill things like webhosting, mail pickup, and mailing lists. These things usually run without complaint, but occasionally something bizarre happens.
Like today. One of the mailing lists I host kept having mail bounce to one particular yahoo user, typically with the complaint that my server was trying to relay mail through yahoo to an, um, yahoo user. Other yahoo users were not having this trouble, so I kept blowing off the complaints as "it's yahoo's fault, not mine". But eventually the complaints got to the point where I had to do some investigation, and discovered, to my intense amusement, that the stupid mail server that yahoo uses doesn't know how to parse domain names, so mail addressed to luser@yahoo.com is delivered, but mail addressed to luser@yahoo.com. is not.
Yahoo.com and yahoo.com. are, as far as the dns is concerned, the same site. And so any mail server will cheerfully relay the mail off to one of the mxes. Mail servers like Postoffice won't even rewrite the outgoing mail headers, because they (foolishly?) believe that if a site is going to advertise itself in dns, it will accept mail addressed to it no matter whether the domain is typed in UPPERCASE or has a period at the end. So it doesn't seem like rocket science to accept mail that comes into the mail server if the domain is UPPERCASE or has a period at the end.
But this is yahoo, an organization that has an abuse desk that uses the Evil Party playbook when dealing with spam; there are large swaths of the yahoo IP space that I don't accept mail from any more because when I forward a piece of spam to their "abuse" desk, complete with circles and arrows and a paragraph on the back pointing out that the spam came from their address space, their reaction is "golly, it's horrible that you've got spam, but something.something.yahoo.com doesn't belong to us."
So they've got mail servers that won't accept mail addressed to valid users in their domain? Well, yeah, that's about par for the course. I'll bet they consider blocking valid domain names as being part of their antispam system, too.