One of the big reasons that emails go to spam is that spam filtering has become more rigorous. Email providers are simply cracking down on spam. However, the filters arent 100% accurate, so sometimes legit emails go to spam too.
Check out this guide from Google for tips on how to stop emails from being accidentally marked as spam: http://goo.gl/kxRa9q
Also, we have a resource on how to whitelist emails here: https://buyandsellabusiness.com/how-to-whitelist-emails
Hope that helps.