MTA's poor reputation "
Resolution:
Tips to solve this error:
1. Configure RDNS (reverse DNS) for your mail server.
2. Some experts recommend configuring SPF records for all domains that respond to the server's IP.
You can find more information here:
http://www.openspf.org/SPF_Record_Syntax
http://old.openspf.org/dns.html
http://old.openspf.org/wizard.html
3.
Ensure that your server is properly hardened, secured and updated to avoid possible exploits that would allow hackers to run spam campaigns through your hardware.
Make sure that your server is properly reinforced, protected and updated to avoid possible exploits that allow hackers to execute the sending of spam through your server.
4. Analyze your computer with anti-malware and antirootkit software.
5. Check the email logs and verify that your server is not sending spam of any kind.
Below I give you some additional information:
http://www.reputationauthority.org/index.php
http://www.trustedsource.org/