Typically IPs get blacklisted when the the outbound relay servers send out spam and this is detected/noticed by worldwide blacklist resources. In the case of Mithi SkyConnect, all outbound traffic is routed via SMX and is scanned for virus and spam to ensure that only clean mail leave the periphery. This reduces the chance of IP blacklisting down to near zero.
Since we moved to this architecture, we’ve stopped worrying about blacklisting of our outbound IPs. Another thing we care about is the DNS hygiene of our customers. We ensure proper configuration of SPF, DKIM etc to ensure that the reputation of our customers’ domains is very high.