You may send to 400 recipients (email addresses) per hour.  RCN does not allow sending any unsolicited advertising. 

From the Internet Access Agreement:

1. Prohibited / Abusive Activities

(d) Unsolicited Advertising. You agree not to post or transmit any unsolicited advertising, promotional materials, or other forms of solicitation to other subscribers, individuals, or entities, except in those areas (e.g., classified advertisement areas) that are designated for such a purpose. You further agree not to involve or associate RCN in any way with the posting or transmission of unsolicited advertising, promotional materials, or other forms of solicitation, including but not limited to unsolicited advertisements sent from another service provider advertising an RCN-hosted web page, and unsolicited advertisements sent from another service provider which request that replies be sent to an RCN email address.