Do you know how many people use outlook, hotmail, live etc.?
If you care about your custumers you fix this. Either check if MS put c-cex on a blacklist and change the ip, or use a SMTP mail server.
Not much and we are resolving each e-mail change request tickets very fast. Our sending mail server is set perfectly and sending mail to all other mail services without any problem. It's microsoft mail service specific issue. So it's not a big issue. Everybody can get new mailbox from other service in seconds.