Blacklist & block list for emails/domian/IP

I run an IT company, we are busy : We support Many companies and run a lot of projects each year. We support many exchange server.
We are a full control company: Sale of Servers to MSP supporting the companies.

I have noticed that all the companies who were on exchange 2003 etc (older platform) did not have any issue and never got black/block listed.
As we move along: We use SSL Certificates from Godaddy, 2013/2010 Exchange : 4443 Port forwarding to Exchange, Properly configured exchange and also Barracuda as a SPAM filtering service for all clients as well.
We go to the point that Port 25 is blocked on the firewall.
Black/Block lists are taking toll on our support.

Is there something we are missing?