A number of hosts these days offer unlimited email accounts. While this can be very useful for websites with many staff members, you may not be able to send out bulk email. In an effort to conserve precious bandwidth and CPU usage, some web hosts have banned the sending of bulk email to large numbers of users.
While such a measure does help prevent abuse, it can be a huge hassle to administrators of sites with thousands of registered users. There are third-party services out there that will handle bulk email separately, but these cost extra and do not integrate with sites.
Your best bet is to read the fine print before signing up with a host and predict your needs ahead of time. If your host restricts bulk email and you want to send out a gardening newsletter monthly to twelve people, you should be fine. Multiple that number by 100, however, and you could run into problems.