You need to check for the tree most common issues:
1. Check rDNS if its properly set
2. Check that you have enabled DKIM
3. Check that you are not blacklisted

1. Check rDNS if its properly set
rDNS must be set as your hostname, you need to have a valid SubDomain for your hostname, also this SubDomain needs to have a proper A record in the DNS zone.
rDNS can be set only by your hosting provider, so you will need to contact them.

You can check if you have a proper hostname and rDNS bellow, also you can check your mail server reputation.
http://www.senderbase.org/

2. Check that you have enabled DKIM
Rebuild the Mail Server using "Postfix Manager" in CWP.admin with DKIM & SPF.
Using DKIM Manager add DKIM key for each domain you need.

To test the set-up, simply send an email to check-auth@verifier.port25.com and you should receive back an email containing something like "DKIM check: pass"

3. Check that you are not blacklisted
Check if your Mail Server IP is blacklisted by using the link bellow:
http://mxtoolbox.com/blacklists.aspx

4. Check if you have DMARC records set
More info:
http://www.gettingemaildelivered.com/how-to-set-up-dmarc-email-authentication