Make sure you have SPF and DKIM setup. Since you are mailing from your own server, this is highly recommended.
Here is a link to Namecheap with screenshots. I couldn't explain it better:
https://www.namecheap.com/support/knowledgebase/article.aspx/9214/31/email-authentication-tool-in-cpanel-spf-records
Also, there are various checkers out there that well validate your SPF, DKIM, and check other things, and give it a score.
http://www.port25.com/support/authentication-center/email-verification/
https://www.mail-tester.com
http://dkimvalidator.com
The only things you can do when mailing from your own server. Use SPF and DKIM. Stay away from spammy words. One wrong word can make the difference of spam or inbox. Have a baseline email and baseline subject for testing. I had a slogan of mine that landed me in the spam folder, so I had to change it because I wanted it in the signature of my emails. Using certain link shorteners could land you in the spam folder. I am not saying they do, but using bit.ly or tinyurl links could land you there. Using tinypic or another image hosting service could. Your domain name could be the problem! The from name..
Remember, every word (whether it's in your email address, domain name, hostname, subject, or the body of the email) is looked at.
As well as your server's IP address. Is it on any blacklists? Check it with http://mxtoolbox.com/blacklists.aspx
Without one of us actually getting on your server and sending dozens of test email to see what is actually causing it, I can only tell you the dozens of things to check.
With all that said, mailing off your server sucks and I don't recommend it. I suggest using SendGrid or Mailgun. I had a client landing in the spam folder, on a new server, clean IP, new domain, everything. I put him on SendGrid and he wasn instantly going into the inbox.
For development, or a low level site, mailing off your server is fine if your hitting the inbox most of the time. But anyone that takes their site serious and the emails hitting the inbox is important, use a 3rd party. Most servers (from GoDaddy, Hostgator, etc.) are not for sending emails to tens of thousands of customers. They are web servers, not mailing (MX) servers, and they will tell you that it isn't their problem.
Update: I forgot to mention reverse DNS. You should have rDNS setup, and that is done by your upstream. So you would need to contact your webhost and ask them to setup "Reverse DNS" on your server.