Hello,
Greetings!!!!
From your project description, what I understand, your client website doing spamming which put your server in blacklist? Please correct me if I'm wrong.
If you know those particular websites getting getting hacked, then I would recommend to check the code with the developer.
As far as server security and hardening is concerned, then let me know so can proceed accordingly.
To know about me, kindly go through my profile.
Thank you,
Jagdeep Gill
Skype Jaggyd86