Receive error: Target mailbox doesn't have an SMTP proxy matching '' when move mailbox to Office 365

Recently I was working on Exchange Hybrid Deployment for one of our customer. The Hybrid Configuration process itself went smoothly. No errors for HCW. 

The problems came when I tried to move mailbox to Exchange Online. The migration fails with error:
Target mailbox doesn't have an SMTP proxy matching '<domain>'"

To check that I ran the following command against the On-premise Exchange.

Get-Mailbox "O365 Test5" | fl and check the EmailAddress filed

Then I ran get-mailuser in Exchange Online. Because the mailbox hasn't been migrated, the user still show up as Mailuser(Contact) in Office 365.

Get-MailUser "o365 test6" |fl

Noticed the proxy address "" does not show up into the Azure AD object. The user object in Office 365  only has our primary SMTP address "" and "" stamped. "" is missing from the address list.

After review the DirSync rules, I notice for user object, the proxyAddress attribute does not have an "Import" rule associated with it. As a test I manually added a Import rule for "proxyAddresses".
The rule details are:

After kicked off a full sync by run "Start-OnlineCoexistenceSync". 

"get-mailuser" was able to return with the SMTP addresses "". Mailbox migration to Office 365 went successfully after that.


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