I used to work for a major shared environment web hosting company that also hosted mail for its customers and the mail was the absolute worst. Both in terms of day to day support of users wanting to connect mail clients and in the bigger scope of keeping our mail gateways in good reputation on global blacklists. All it took was a couple bad actors to ruin mail reputation from an entire cluster of servers, and in shared hosting you’re bound to have well more than a few bad actors.

We had methods in place to try to keep it in check, but it was like herding cats. I left that company several years ago but even then they had been trying to ramp down and discourage mail hosting by offering Google Apps, not sure if they still host mail or not.