Emails going to Spam folder have many reasons, mostly is due email content and mistakes made by the receiving email server.
DNS Exit keeps all IPs clean and will stop using the IP once it is blacklisted. IPs in our mail relay system are usually not the cause emails ending at Spam folder. DNS Exit maintains about 1000 IPs from more than 30 providers and we got enough room to switch to other IPs is once a IP is blacklisted. There is no much we can help to improve on the problem; however, there are a few things you can do to improve the delivery rate to INBOX:
1. Make sure the sender's domain has SPF defined and the SPF includes our SPF with string:
2. 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.
3. 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.
4. 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.