I'd have to wonder if it's due to the message failing reverse DNS checks on some of the hosts. If a email is sent as
yoursar@gmail.com (made up of course), but it originated from the MM servers, the receiving host will often do a reverse check and realize that it wasn't truly sent from a gmail server based on the IP address. Often that's indicative of someone spamming. Obviously there's only so much that can be done to deal with that since it's done on the yahoo servers and not necessarily the recipient email account. Even whitelisting wouldn't resolve the issue since that just says that the sender
yoursar@gmail.com is known to me and isn't spamming, but since it's failing the reverse dns check, it's considered to be not reliable. Hopefully that all makes sense. I receive MM messages in my google apps account (domain email hosted on gmail servers) without an issues, but I can't say the same for yahoo. Do you know if anyone on your team using yahoo receives messages from MM?
