Disabled account OU

Your employee’s on-premises AD account is synchronized again, but the former mailbox appears to be a separate cloud shared-mailbox object. The issue is object identity: Exchange cannot link two Entra objects merely because an SMTP address or username matches. Do not delete either object. Compare the synchronized user’s Entra Object ID with the shared mailbox’s ExternalDirectoryObjectId, and record the ExchangeGuid and SMTP address. If the shared mailbox remains anchored to the same user object, convert it back to a regular mailbox, assign an Exchange Online license, and allow directory synchronization to finish. If the IDs differ, do not hard-match or change ImmutableId blindly; that can disconnect or duplicate the mailbox. Grant the returning user Full Access and Send As temporarily, then open a Microsoft 365 support case to validate the reassociation path using both object IDs and ExchangeGuid. A tenant-specific merge cannot be confirmed safely from this description alone.