A domain key is a huge help, but they don't skip the filters 100% and guarentee anything you send will get through.
Your mail could still be blocked for certain keywords or addresses in the links.
Best way si to play around with your letter content until you know it gets through to inbox, then keep checking each day (ie. send to a test yahoo account), so if something changes, you can adapt right away.
Getting mail through is a day-to-day task, but it can definately pay off
Good luck.
Also, make sure your mailserver is configured to use your domain key properly. You can test this by sending yourself a mail, and you should see in the header "This email is confirmed to be sent from xxx.com"