Gmail works hard to keep spammers from using Gmail addresses. As part of this effort, we refuse to create addresses that we believe will be used for spam. Unfortunately, we may sometimes refuse legitimate signups. If you've been stopped in error, we're sorry for the inconvenience.
Gmail weighs several factors when evaluating registration attempts, including the number of addresses that have been created from that location. Users trying to create a lot of addresses at once, like for a class or group, may hit this limit.
If you routinely need to create addresses in bulk on behalf of other users, please consider using Google Apps.