Gmail takes these measures in an attempt to keep spammers off of their system. Sending bulk e-mail or sending to many addresses that don't accept your e-mail are telltale signs of spamming activity.
If you need to communicate with the same set of people often, Google recommends using their Google Groups service to create discussions, webpages, and even share files. If you run a server, you could utilize a listserver (so you can send an e-mail to one e-mail address to make Gmail happy, and then the listserver would send the message to all the members of that listserver).
[Via Digital Inspiration]
