Microsoft has increased the maximum number of domains per tenant allowed in Office 365 by 50%, from 600 domains to 900 domains. The increase is automatic, so administrators do not need to do anything to take advantage of this improvement. Up to 900 domains can be added from the Office 365 admin portal or via remote PowerShell.
About The Author
Nuno Mota is an Exchange MVP working as a Microsoft Messaging Specialist for a financial institution. He is passionate about Exchange, Lync, Active Directory, PowerShell, and Security. Besides writing his personal Exchange blog, LetsExchange.blogspot.com, he regularly participates in the Exchange TechNet forums and is the author of the book “Microsoft Exchange Server 2013 High Availability.”