January 13th, 2015 until February 13th, 2099
I've relaxed the new user PM options. New registered users can now send and receive PMs. They can only store 10 in their inbox and can only send message to two recipients at once and only one message may be sent every 15 minutes.
This should be reasonable for folks trying to use the forums normally but will hopefully be a strong deterrent to those trying to spam everyone. I hope.
As before this only applies to those with a post count below a certain threshold. This is similar to the post count limit for signatures and other features.
The post count threshold is not an invitation to post frivolously to gain access to the features. That, too, is spamming. But the threshold is low enough that normal users should be able to get there with minimal participation.