If it is going to their spam folders, then you need to insure a few things are set correctly first. You want to make sure your reverse DNS maps back to your mailserver and that you have an spf record set (DA does that by default). Try going to dnsreports.com and put one of your domain names in, it will tell you if it sees a problem. Another test to do, is the "Spam database lookup" on dnsstuff.com, if you are listed in red in anything other than the ones marked "should not be used" then that could be your problem.
If all of that is clean, then you need to speak with each of the big Email providers to whitelist your host. Most will do that that after a few emails, except for Hotmail and MSN (they require you to pay a few thousand dollars a year to be whitelisted).