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.

If the emails sent to the popular email providers like yahoo, gmail, hotmail, aol and they ends up at Spam folder, make sure you click on the "NonSpam" button to tell the system that the email is not Spam. The will help those email system to identify our emails and avoid same mistakes for future emails.

Try to avoid the email spam trigger words like job, viagra, clearance, order, fast cach, etc. in your email. You may refer to this link for such spam trigger words.

It is common that the receiving server may blacklist the sender's domain or email address. You may want to send an email to this address from another domain address to see how it ends. If they blacklist your email, there is no way to avoid that.