There might be cases where in Exchange hybrid deployments where Exchange Online and on-premises users have the same e-mail namespace (such as firstname.lastname@example.org), Office 365 users are unable to send e-mails to on-premises users and receive a nondelivery report (NDR) error message similar to:
#550 5.1.1 RESOLVER.ADR.ExRecipNotFound; not found ##
Amongst other possible reasons, this can be caused if the domain set up in the hybrid deployment is not configured as a shared domain in Office 365. To correct this problem, follow these steps using the Exchange Admin Center in the new Office 365:
1. Sign in to the Office 365 portal as a global admin;
2. In the header, click Admin, and then click Exchange;
3. In the left navigation pane of the Exchange Admin Center, click mail flow and then click accepted domains;
4. Select the domain that is set up for the hybrid deployment, and then click Details;
5. Select Shared, and then click save.
The EAC the domains that you added to your account through the Microsoft Office 365 portal. It lets you manage how messages are delivered. In a hybrid scenario, Exchange Online must be set up correctly so that when a cloud-based user sends an e-mail to an on-premises user, Exchange Online routes the e-mail to the on-premises messaging environment.