Sending email to multiple addressees

I am trying to send an email to about 30 people I work with and it refuses to go through. I have an email group in my address book composed of about 30 addresses. When I "send" it, it refuses to go through. Before this month it worked fine. I can still send emails to individual addresses and small groups (6 or so). I have contacted my ISP and they are unable to locate any problem from their end. HELP!