Office 365 users gets error code 5.1.11 bounce back emails randomly.
Status code: 550 5.1.11
The recipient email address is a LegacyExchangeDN address, which isn’t used by the Office 365 service. You might see this error if you’ve migrated your organization’s email from on-premises 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 Auto-Complete List from the user’s Outlook or Outlook on the web doesn’t solve the problem, try to clear the related LegacyExchangeDN address from your on-premises Active Directory. Then synchronize the directory again.
Did removed auto-complete entry several times and still has issues.
Why this is happening and what is the fix?