I've seen that there's no such feature as a pm system as stated on the answer to this question and I perfectly understand why;
The problem is that a new user has no way of replying to inbox since:
- he/she can't post comments until obtaining 50 reputation ( and it would be unadvisable to reply there, because it would just clutter up a space reserved to useful comments );
- he/she can't use chat either since it has some other reputation requirement.
I sincerely can't understand this last point, and I'm not referring to the inability to contact a user first ( because that would open up the ground for spammers ), but rather to the disallowance on replying to an inbox.
I think his particular aspect might be something worth looking into and that there's room for a new useful feature.
Sorry for the somewhat muddled bits of this post.