After you migrate mailboxes from your on-premises environment to Microsoft Office 365 in a hybrid deployment, the From field of email messages that are sent from the on-premises environment to Office 365 doesn't show display names. Instead, the From field shows email addresses.
This issue occurs if the hybrid deployment is set up incorrectly.
To verify that you're experiencing this issue, examine the email header of an email message that was sent from the on-premises user account. Typically, X-MS-Exchange-Organization-AuthAs should be listed as "Internal." If X-MS-Exchange-Organization-AuthAs is listed as "anonymous" or if it's missing, this indicates an incorrect configuration or an incorrect mail route.
To fix this issue, follow these steps:
Check the mail route.
The simplest route is Exchange 2010 mailbox server to Exchange 2010 hub server (hybrid server) to the Exchange Online Protection (EOP) inbound connector to Exchange Online. Make sure that there are no unnecessary network devices such as anti-spam gateway devices between the Exchange 2010 hub server (hybrid server) and EOP. Those devices could remove the necessary header.
Check the remote domain of the on-premises Exchange server. To do this, follow these steps:
In Exchange Management Shell, run the following PowerShell command:
In the output, make sure that the TrustedMailnboundEnabled attribute is set to True.
If the attribute in step 3C isn't set to True, use the Set-RemoteDomain command to change the value to True.
Make sure that Transport Layer Security (TLS) is implemented and enabled in both environments and that the fully qualified domain name (FQDN) is set correctly. Check the on-premises send connector and the EOP inbound connector by using the Exchange Server Deployment Assistant at the following Microsoft website:
Check the Exchange certificate of the send connector on the on-premises Exchange servers that are responsible for delivering mail to EOP. The Exchange certificate should have Simple Mail Transfer Protocol (SMTP) enabled and should match the FQDN of the send connector.
If the issue persists after you follow these steps, contact Office 365 Support.
For more info, see the following post in the Office 365 Community: