Fix Email Service on Linux CentOS Server

I have a Linux CentOS Server running the latest version. I host domains and other website content. In addition to these items, I run a mail server through this machine. I need to SECURE this server and the mail server within. I constantly receive "delivery failure status notifications" stating emails that were sent from one of my domains was not accepted by Google, Yahoo, etc. I need to find and fix this vulnerability so it doesn’t happen again. Furthermore, I need to change the IP address associated with the server for sending emails. As the old IP address has now been blocked by several ISPs. Lastly, fix SPFs for EACH email account on the system. I’d like the domains to be able to send and receive emails without issues from ISPs. Please provide a price and the amount of time it will take you to fix these issues. Once we agree on the price and timeline, you will need to send your Public SSH Key so I can provide you access to the server. If you have questions, please PM me, thanks.