First you'd have to look at why they are going to spam. If it's content, then you have to look at what's in the content that is making them go to spam. If it's because your server is on a RBL (realtime black list) then you have to resolve the issue that got you on the black list. Finally (what every domain owner can do) is to setup SPF and DKIM records. Those, in themselves, won't guarantee it won't go to spam, but it will help the receiving system determine that they came from a legitimate source.