550 5.1.10 RESOLVER.ADR.RecipientNotFound When sending email from Exchange Online to On-Premises Exchange Server

There are different reasons for email delivery failure from exchange online to on-premises exchange server. Error 550 5.1.10 is one of them.

Problem: 

An Office 365 user is unable to send email messages to on-premises Exchange users in an Exchange hybrid deployment.

Cause:

This problem occurs if the user was created in Office 365. In this scenario, the on-premises Exchange environment has no object to reference for the user. Therefore, all queries for that SMTP address fail.

Solution:

To resolve this problem, create a remote mailbox in the on-premises environment and then wait for or force directory synchronization to occur. To do this, follow these steps:

1. Create an on-premises object for the cloud mailbox by using the New-RemoteMailbox cmdlet in the Exchange Management Shell.

   Note: This object must have the same name, alias, and user principal name (UPN) as the cloud mailbox. 

2. Set the ExchangeGuid property on the new on-premises object that you created in step 1 to match the cloud mailbox. To do this, follow these steps:

   A. Connect to Exchange Online by using a remote PowerShell.

   B. Use the Get-Mailbox cmdlet to retrieve the value of the ExchangeGuid property of the cloud mailbox.

   For example, run the following command:

   Get-Mailbox <MailboxName>| FL ExchangeGuid
   

   C. Open the Exchange Management Shell on the on-premises Exchange server.

   D. Use the Set-RemoteMailbox cmdlet to set the value of the ExchangeGuid property on the on-premises object to the value that you retrieved in step 2B.

   For example, run the following command:

   Set-RemoteMailbox <MailboxName> -ExchangeGuid <GUID>
   

3. Wait for directory synchronization to occur. Or, force directory synchronization.

      4. Make sure that the Office 365 user object is displayed as Synced with Active Directory.