I assumed it was deliberate? I used to experience it myself too some years back, when I used to engage in private messages on here. It's quirky though, I agree. On twitter (for an example) you can only direct message someone if you follow them (ie, you are a contact) but with Moodle, someone can message you if your messages are open to all, but you cannot reply if they have it set to only contacts.
It seems to be that if you can message someone then that person ought to be able to reply - regardless of your privacy settings. It seems like a logical exception.
I vote for bug! That sort of thing is very much the abilities a 'spammer' has discovered they can employ ... only it's right inside Moodle??!!!!
2 cent vote/opinion.
'spirit of sharing', Ken
It is a setting available on each users Preferences.
