Exchange 2007 Mass Email Bouncebacks

I work in a small corporate environment on a all Microsoft server network/domain.
We have Exchange 2007 SR1 on a Windows Server 2008R2 server.

Our CEO sent out a mass email. Same email to almost 300 addresses. All the emails came back.
My exchange server is set to allow 5000 addresses and a 130MB pipe.
I sent the email out in smaller groups using her account and only 24 bounced.

I suspect my ISP has a spam filter in place. My director doesn't believe so. He thinks it's the server.

Are ISP's (like XO) setting up spam filters? It there a way around them?

Thank you.

 

Some anti spam products, and SBS 2011 use directory harvesting, if a non-existent email exists in an email addressed to multiple people at the same organization, it will bounce the whole email. The number of non-existant email addresses that can be allowed can be increased, but by default it is 1 on some cases.