Exchange not delivering newsletters

Exchange 5.5 with pop3 conector for internet mail.

My exchange server isn’t delivering some newsletters that come in via internet e-mail. Some newsletters (like tech republic) get to the correct users, but many end up in the administrators mail box as undeliverable. I think it has something to do with the way the mail is formated by the sender, but I can’t find anything specific.

Any one have the same problem or any idea on how to fix it?