The message couldn’t be delivered because the email address wasn't recognized. To try to fix the problem, clear the recipient AutoComplete cache in Outlook or Outlook Web App by doing the following:
1. Click New mail (or New Email).
2. In the To box, start typing the recipient's name or email address until the recipient appears in the drop-down list.
3. Use the Down Arrow and Up Arrow keys to select the recipient, then press the Delete key.
4. Resend your message – delete and retype the recipient’s name or email address before sending it.
If the problem continues, forward this message to your email admin.
For Email Administrators
The recipient email address is a LegacyExchangeDN address, an email address format which isn’t used or supported by the Office 365 service. You might see this error if you've migrated from an on-premises deployment to the cloud, or if your organization has a hybrid configuration and you synchronize your on-premises directory with Office 365. If clearing the recipient AutoComplete cache from the user's Outlook or Outlook Web App client doesn't fix the problem, try to clear the related LegacyExchangeDN address from your on-premises Active Directory. You can do this by using the LegacyExchangeDN Utility. Then re-synch the directory