Use the primary SMTP address for on-premises mailboxes and not user IDs from the %BRAND_OFFICE_365_SHORT% portal. For example, ...

Use the primary SMTP address for on-premises mailboxes and not user IDs from the %BRAND_OFFICE_365_SHORT% portal. For example, if the on-premises domain is named contoso.com but the %BRAND_OFFICE_365_SHORT% email domain is named service.contoso.com, you would use the contoso.com domain name for email addresses in the CSV file.