As the title says, when is one enabled to receive and/or send private messages here?

You have to be a member for a number of days. It prevents "drive by" messages from spammers. If you got instant access, then people would sign up just to send/spam people messages. After waiting X days, a spammer will have moved on to another site and forgotten about CathInfo, while a real member would still be here, and merely inconvenienced a bit.

When I was an admin at another site, there were Indian and Pakistani spammers who would quietly register, lurk for about six months, then post 10 posts, which did not draw any suspicion. With ten posts, they would have access to Private Messaging. Then the nightmares would start.

Unless people reported those PMs, these spammers would send PM advertisement for products, porn, etc. In addition, they would change their signature to a link while they softly advertised natural products (supplements and cosmetics) in their posts.

It seemed like a coordinated attack, because we would have about ten of these spammers suddenly activate their ability to PM each day for a week. After we had banned 70 of these gems, then there would be blessed silence. However, in a week or two, there would be another wave of them with more in the wings.

Since these were not bots, but real people, they could answer most of the CAPTA questions and easily register and wait.