Lazy thoughts on hypothetical private messaging system

The idea of adding ability to exchange short messages between users (at least to be able to share email address or other way of more convenient communcation) broods lazily in my mind for some months.

Besides my somewhat laziness the issue which prevented me from "just go and do it" was the recollection of receiving annoying "Hi Sir please send me your code for task XYZ" sometimes (at codeforces account).

However slowly I come up with simple idea of amending this - to allow user to limit rank of those who can send messages to him/her. E.g. by default it could be (as with forums) that no messages could be sent by users with less then 5 problems solved. However one may set limit that incoming messages are allowed only from peers with, say, 100+ solutions.

Or even 100500 (no messages allowed at all).

This a bit confuses me because it sounds somewhat silly, but looks more "just" than filtering messages by country or personally. Probably also personal blacklists should be implemented.

Though it is not expected the feature is going to be used much, so perhaps not too many thoughts and efforts are needed for initial implementation :)

As this anyway somehow touches people's privacy, I would be glad to have someone other's ideas/hints/thoughts. Perhaps there are criteria better than num-tasks-solved for filtering...

Thanks in advance.

P.S. one of the issues, is, by the way, I'm not sure about implementing something like email notifications about new messages (as due to GDPR etc I feel bit reluctant to collect too many email addresses). But at least some visible notifications in user interface we should have...

This is something never occurred to my mind but definitely with obvious advantages, thanks!

Though this makes obvious the necessity of some notifications for forum, e.g. marking which topics have new messages for given user or some alerts that given forum has new messages... Shall try to think of appropriate logic.