stop emails from my server being marked as spam

Hi, I need help with our settings - at the moment emails sent from our server are marked as spam on some services, i.e. gmail, As we don't send spam and never were sending spam I believe it should be only a matter or putting the right settings in domain control ? I believe this should be easy to do for someone who understands it... So budget for that is approx. $30, Ideally this should be done very quick. Many thanks,