Emails going to Spam folder have many reasons, mostly is due email content and mistakes made by the receiving email server. However, SPF is a MUST for your email to deliver to destination email servers. Now Gmail will either reject or toast your email to Junk if your domain does not have SPF matched. DKIM is also very important factor for email delivery rate. You need to make sure you have done the followings:

1) Make sure the sender's domain has SPF defined and include our SPF with string: include:dnsexit.com. IF your domain does not have SPF yet, just make your SPF as v=spf1 include:dnsexit.com -all

2) Add a CNAME relay._domainkey.{yourdomain.com} and point to dkim.dnsexit.com. 

After that, login to your account and click on Sender Domains under Mail Relay section right after you login. Add your domains to the Sender Domains list and very SPF and DKIM. After being verified, DKIM signature will be inserted to your email sent through us.