Thanks for the explanations.
We are likely going to turn off Reply by Email for two reasons: (1) threads that go off track because users replying by email haven’t seen the intervening posts, and (2) moderator time cleaning up personal info from email signatures that sneaks through. We would consider still allowing RbE for >TL2 users since they are more trustworthy (if that were an option).
I understand the setting now. I think it would much more clear if the explanatory text read “Disallow reply by email (N) days after email sent. 0 to allow indefinitely.”
On further testing, it does appear that replies by email to PMs from regular users are in fact blocked when the feature is disabled. However, unlike replies to posts, these replies seem to just disappear without generating an error message. If this is correct, there really should be an error message generated so that the user knows that their reply didn’t get posted. I’d be interested to know if you can replicate this.