554 Transaction failed : Cannot send message due to possible abuse; please visit?

The limit for regular email accounts is 100 recipients. Try breaking down your list. Your group should be able to exceed that limit, tho.

554 is supposed to mean that something in your email triggered the spam filter (even tho you weren't spamming).

As of January, 2011, you can send one email to 100 recipients. You can send 500 total emails per day. You are sending too many at once.