I came here because of postal. I find the questions relevant, and would like to propose the following.
I suppose there is a way, using the Discourse Data Explorer, to find who has been receiving email but did not login in over 6 months, and then send them an email asking them whether they’re still interested or do they just ignore email. If after, say, another week, there is no reply, stop sending them email, and eventually require account validation (i.e., suspend the account for inactivity.)
This would certainly save lots of CPU cycles and network bandwidth (and power and water…) not sending useless email over the Internet across all thousands of Discourse instances. 