bonhomme_de_neige writes "Emails and invitations sent to Hotmail from Gmail accounts do not bounce, but nor do they arrive in the recipient's Inbox - they vanish mysteriously into the aether. Joel Johnson writes in his Gizmodo weblog that invitations he sent to a Hotmail address bounced (this even received coverage from ZDNet). Search Engine Roundtable writes that several ISPs are blocking Gmail. It's already well-documented that Yahoo moves Gmail invites into the Bulk Mail folder. I've personally confirmed the Hotmail and Yahoo blocking."
Please note: I've not been able to verify this one way or another.