This is a key measure of the quality of your email list, and of the effectiveness and relevance of your message. Segmenting this metric is really powerful. You can learn whether text messages or messages with images get a higher CTDR. You can compare customers in California, Idaho, and Florida; new and existing customers; or various demographics, etc., etc., etc. This should drive aggressive experimentation of email content / offers / targeting / every facet by your team.