I don't know the number of PMs is limited, but I usually don't use this feature. Is it because the account is too new, so there is this restriction.
AFAIK, buying copper membership will allow newbies to bypass their limitations, and act as a Member. One of them includes sending PM. You can read more about this on copper membership details.
Wow - can we have an option to block messages from copper members?
Unless there is enough case of abuse by a copper member I doubt theymos will apply it.