We have 15+ Unlimited enterprise SMs and made a mistake of upgrading all of our servers to SM 16. Even now we don't know why we did it on every server but it was a mistake we have to live with it.
We COMPLETELY regret the update. It was the worst decision our company made in the 18 years of it's hosting industry presence.
It is a unstable product, has many fundamental bugs that appear on every minor release and if one bug gets fixed you can be sure that another WILL appear.
Our support is wasting many hours daily explaining to our customers that we have to wait for a possible future update that will hopefully get all the issues fixed.
But to be fair the issues are getting fixed by SmaterTools and in the near future we will mostly likely have a wonderful mail server but it isn't now.
In the end I urge to wait until there is no more bug/problem report on SM16 and only then upgrade your Nodes.