It's 100, but that's per running hour (as in any 60 consecutive minutes), so if you're trying to run at the limit and your timing is just a little bit off, you're going to be grey-listed. Remember that your crons are competing for resources with other crons and page requests, etc., so there will be some jitter in the timimg, and that mail will go into an outbound queue before they're counted, spam-checked and sent, so the period of time over which you send the emails from your account will not necessarily be the same period of time over which they're inspected by the system. Realistically, anything more than 50/hr is gambling. Even more realistically, mass mailing from a Free Hosting account is not a great idea; you can use free outside services like MailChimp for things like newsletters if you need to get a wide-ish distribution out over a short period of time. (The free tier of these services tends to be restricted, but if your community is sized to fit on a Free Hosting account at all, you'll probably fit within those restrictions.)
