When an Office 365 user applies a forwarding rule to automatically forward email messages to more than 10 recipients, the rule doesn't work and the messages aren't forwarded.
This behavior occurs because a forwarding rule can send mail to no more than 10 recipients.
To work around this behavior, use a distribution group in the forwarding rule.
Article ID: 3133729 - Last Review: Dec 29, 2016 - Revision: 1
Microsoft Exchange Online