What version are you running?
2.5
What's the URL of the page this enhancement relates to, if any?
https://github.com/reviewboard/reviewboard/blob/master/reviewboard/notifications/email.py
Describe the enhancement and the motivation for it.
Now that we have X-ReviewBoard-ShipIt and X-ReviewBoard-ShipIt-Only to help filter [Ship It] messages, it probably makes sense to add something like X-ReviewBoard-Conversation for those who want to filter only for messages containing review responses (or other conversation).
What operating system are you using? What browser?

Please provide any additional information below.
We have found X-ReviewBoard-ShipIt very useful for picking out e-mails that contain actionable content (i.e. [Ship It]). In our group, the next most actionable content is "someone has commented on code / someone has responded to  comments"
If there were something like X-ReviewBoard-Conversation or X-ReviewBoard-Comments to mark when a conversation is taking place (as opposed to just an updated diff/description, etc.) that would cover all of our most common e-mail filter requests.