There is no accurate way to measure "open rate" because it's a stupid invasive metric. Anyone with half a brain blocks HTML rendering or at the very least remote images in images. The way to use this as intended is to request a read-receipt, and that can be turned off because nobody cares to populate your marketing telemetry. Use a call-to-action that leads to a tracked page that gauges users interest instead and you'll get users who actually might care about your comms.