Well, there's plenty of discussions of multiple PTR records under a single IP such as: https://serverfault.com/questions/618700/why-multiple-ptr-records-in-dns-is-not-recommended

In general it remains not recommended as simply put: alot of software just expects a single name back, not multiple.

However SolidCP is opensource, so anyone can contribute reverse dns support (or dkim. dmarc is already supported since it's just a txt record much like spf).