Hi,
We are fast growing hosting company in India. We have expertise in Linux and Windows server setup with management.
And work with different hosting panels like cpanel, plesk , Dot net panel, webmin etc..
We will check the mail server and then let you know the loop holes behind the issue.
We will implement below techniques for proper functioning of mail server-
Techniques used:-
1. Ip rotation -> Prevent blacklisting
2. Spf , domain keys enabled-> Increase inbox delivery
3. Reverse dns enabled -> Increase inbox delivery
4. Open relay disabled -> For Mailing server security
Thanks