New registrations password email doesn't arrive

My forum has been up for months now and there's hasn't been any new thread or anything except my own posts. Since my site is not that popular I thought there is really no one who post. I can also see some new registrations here and there.

But now, users informed me they didn't get any email regarding their account password. In short, they are able to register but unable to login since they don't have their passwords.

Is the functionality dependent on SMTP? Since my web host disabled SMTP as I'm only using their free service.

Thanks!

NOTE: My site is http://pw.rob-soft.com/forums