PS - The information about the 5 posts limit is listed in the community guidelines when they join. Assuming they've read what they agreed to when they joined, they should see it there:

We haven't listed the number on purpose, as we don't want a human spammer to know exactly how many posts they need approved before they have free reign. Spam is a huge problem in running forums, and the fact that you see virtually zero such spam here is a testament to the procedures we've put in place.

DocJohn