How did I receive a "Mail rate exceeded limit" error?

How did I receive a "Mail rate exceeded limit" error?

I have been out of the office and haven't sent any emails the last two days, then today I sent one message to a list of 200 people, and about 150 came back with " 550 5.4.6Mail rate exceeded limit" - What is the limit to send per day, I thought it was 250?  The address I am sending from is newsletter@landlinenews.com. 

Kyle