Before you create and configure a hybrid deployment using Microsoft Exchange Server 2013 and the Hybrid Configuration wizard, your existing on-premises Exchange organization must meet certain requirements.
If you don't meet these requirements, you won't be able to complete the steps within the Hybrid Configuration wizard and you won't be able to configure a hybrid deployment between your on-premises Exchange organization and the Exchange Online organization in Microsoft Office 365.
Read about the required prerequisites for...
- On-premises Exchange organization
- Office 365 for enterprises
- Custom domains
- Active Directory synchronization
- Autodiscover DNS records
- Certificates
- EdgeSync
...in the full article here