Why using Custom Outlook forms and HTTP scenarios not recommended?
In Outlook, custom Outlook forms are not supported for use with HTTP-based mail services (such as Microsoft Hotmail). Outlook does not prevent you from using custom form features if you are using an HTTP-based mail service. However, some features do not work correctly. The features do not work correctly because HTTP-based mail messages are stored in a read-only state on the server. Therefore, we recommend that you do not use custom Outlook forms if you are using an HTTP-based mail service.