How to use SMTP matching to match on-premises user accounts to Office 365 user accounts for directory synchronization

SMTP matching limitations

• SMTP matching can be run on user accounts that have a Microsoft Exchange Online email address. For mail-enabled groups and contacts, SMTP matching (Soft match) is supported based on proxy addresses. For detailed information, refer to the "Hard-match vs Soft-match" section of the following Microsoft Azure article: 
  
  Azure AD Connect: When you have an existent tenant
  
  Note This doesn't mean the user must be licensed for Exchange Online. This means that a mailbox that has a primary email address must exist in Exchange Online for SMTP matching to work correctly.
• SMTP matching can be used only one time for user accounts that were originally authored by using Office 365 management tools. After that, the Office 365 user account is bound to the on-premises user by an immutable identity value instead of a primary SMTP address.
• The cloud user’s primary SMTP address can't be updated during the SMTP matching process because the primary SMTP address is the value that is used to link the on-premises user to the cloud user.
• SMTP addresses are considered unique values. Make sure that no two users have the same SMTP address. Otherwise, the sync will fail and you may receive an error message that resembles the following:
  Unable to update this object because the following attributes associated with this object have values that may already be associated with another object in your local directory services: [ProxyAddresses SMTP:john@contoso.com;]. Correct or remove the duplicate values in your local directory.

How to use SMTP matching to match an on-premises user to a cloud identity

1. Obtain the primary SMTP address of the target Office 365 user account. To do this, follow these steps:
   1. Sign in to the Office 365 portal as a global admin.
   2. Click Admin, and then click Exchange to open the Exchange admin center.
   3. In the Exchange admin center, locate and then double-click the user account that you want.
   4. Click email address, and then note the primary SMTP address of the user account.
2. Start Active Directory Users and Computers, and then create a user account in the on-premises domain that matches the target Office 365 user account. For more information about how to do this, go to the following Microsoft TechNet website:
   Create a User Account in Active Directory Users and Computers
3. Set the primary SMTP address of the new user account to match the primary SMTP address that you noted in step 1D.
   
   To do this by using Exchange Management tools, go to the following Microsoft websites:
   • Edit an E-Mail Address Policy
   • Configure User and Resource Mailbox Properties
   If Exchange isn't installed on-premises, you can manage the SMTP address value by using Active Directory Users and Computers:
   1. Right-click the user object, and then click Properties.
   2. On the general tab, update the E-mail field, and then click OK.
4. Synchronize the object with Office 365. To do this, run a force sync on the server that is running Azure AD Connect by using the following cmdlet:

   Start-ADSyncSyncCycle -PolicyType Delta

   For more information, see Azure AD Connect sync: Scheduler.