Yes, it is post-count related, rather than time-related.
The restriction on low-post-count members sending e-mails was put in place to prevent people joining the forum and using the forum's e-mail system to send out spam. You'd be surprised how often that used to happen - and how often it still happens, even with the post-count limit in place.
We don't advertise what the post-count threshold is to activate e-mails, because it's automatic - and we want to discourage the spammers from slamming down x number of "hi there" posts just to boost their post count so they can start spamming straight away.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis