The Office 365 has announced that the federation with Live Messenger will be disabled temporarily in order to solve some issue.

Indeed, users using a vanity domain with Office 365 were not able to access their Live account. So while the team is working to solve this issue, the IM federation with Live will be disabled.

This affect Professionals and Small Business plan, but also Enterprise (less impacted as federation is disabled by default, but some may have enabled it).