Enabling a disabled Mailbox throwing "Error : 72300220 : Mailbox user does not exist "

Question: When enabling a disabled mailbox in HC, it throws "Error : 72300220 : Mailbox user does not exist" or "Disabled Mailbox does not exist".

Answer: When a mailbox is disabled from HC Panel then HC panel disables the mailbox in HC panel’s GUI and at the same time in ECP it sends the mailbox to the disconnected Mailboxes Section.  And it can be re-enabled from the HC panel by simply clicking on Enable Mailbox button in the HC panel’s interface.
Note: Disabled Mailbox is completely removed from Exchange if and only if the retention policy days have passed like if retention policy is for 5 days to retain a disabled mailbox then after passing the limit of 5 days, on the 6th day Exchange disabled mailbox will be completely removed from Exchange Server along-with its emails data.

The latency between Exchange and Active Directory makes Mailbox state to go Out of Sync: As per Microsoft documentation due to latency between Exchange and Active Directory sometimes, the mailboxes that are disabled disappear completely from Exchange and do not show in ECP under Recipients >> Mailboxes Section or under Disconnected Mailboxes in Exchange Admin Center as shown below.

Due to it, when you try to enable the same disabled mailbox from HC panel, it also throws an error  "Error : 72300220 : Mailbox user does not exist" or "Disabled Mailbox does not exist "

Reason Provided by Microsoft for Out of Sync State behavior for disabled mailbox: Details provided by Microsoft on their document about this particular case is as follows :
"It's possible that the store state for a mailbox to become out-of-sync with the state of the corresponding Active Directory user account. This can result from Active Directory replication latency. For example, if a mailbox-enabled user account is disabled in Active Directory but isn't marked as disabled in the Exchange mailbox store. In this case, running the Update-StoreMailboxState will synchronize the mailbox store state with the state of the Active Directory user account and mark the mailbox as disabled in the mailbox store. You can use this command to troubleshoot issues that may be a result when the store state for a mailbox is unexpected or if you suspect that the store state is different than the state for the corresponding Active Directory account". 

To know further details about this explanation, visit Microsoft's Documentation on this page :

Resolving Issue for such Mailboxes: Follow the steps given below to resolve the case.

  1. Open SQL Server Management Studio and connect to HC database table tblExchangeMailboxes (for HC10) or table tblMailboxes (for HC9). Verify that the disabled mailbox entry exists and GUID entry is not showing NULL.
  2. To get GUID for disconnected Mailbox use the following command in EMS :
    Get-MailboxDatabase | Get-MailboxStatistics | Where { $_.DisplayName -eq "HCtestmailbox" } | fl DisplayName,Database,DisconnectReason,DisconnectDate,MailboxGuid
  3. The command in step # 2 will return the mailbox database and GUID, replace these values in the below-mentioned command to instantly update the state of the mailbox in exchange and upon running the command, the mailbox shall instantly list in Exchange disconnected section
    Update-StoreMailboxState -Database “Mailbox Database Name” -Identity “Mailbox GUID”
  4. Now try to enable the user in HC Panel, If it still throws the same error then open the Active directory using command dsa.msc and go to the domain OU then find the user. Username should display like user@domain.com if it is not then rename it and set user@domain.com.

    Note: Exchange retains disconnected mailboxes based on the deleted mailbox retention settings configured for the mailbox database. After the specified mailbox retention period, a disconnected mailbox is purged from the Exchange mailbox database. After the expiry of this period, the mailbox cannot be reinstated through the HC panel.