I’ve created a couple of custom properties on the marketing_email object in an attempt to be able to group our emails with properties instead of a complicated email naming convention. This works well enough within the HubSpot UI, but we’ve discovered that there is no supported public API path to pull custom properties on the marketing_email object, only standard properties (confirmed by HubSpot solutions expert).
The goal is to be able to cross-filter on multiple custom properties within our back-end tables that store our marketing email assets and send activities.