Ahhhhhhhhhhhh, my own personal nightmare. Can you imagine the mess that would be the result those idiots over at Yahoo running MS. Email would be split into two halves, those that were with Yahoo/MS and those that were not.
I had to spend three months getting my schools domain unblocked off Yahoo's mail servers because they arrogantly just locked out the whole New Zealand IP range(apart from Xtra - their puppet) from sending email to Yahoo. Sure there was a solution, you had to apply to them to be able to send email otherwise everything got binned in their junk mail folder this even effected the other major ISPs trying to send to them. This only became an issue when our largest ISA (Xtra) sold its email handling to Yahoo and for months a third of the country couldn't email the others and vise versa. There are still some custom domains that can't send to Yahoo. The problem was worsened by the fact that the users of this provided were the older and less skilled people so they had no idea they could even switch.
If this happened it would be the day that I switch fully to Linux and start my open source crusade against Windows. Or become a builder. :)