For anyone wondering on the same question as I did: Dovecot seems to be a standard IMAP server for now, which seems to be used on majority of servers. It claims to be written with security in mind, which doesn't seem to help to avoid privileges escalations, buffer overflow, crashes (on the same page - below).
Given the dominance of that mail server on the internet, it seems to be a go-to solution for many, just like Ubuntu, referred here above, is; so I wouldn't expect it to be replaced on Cloudron anytime soon.
Given the self-confidence of the authors, that claims that running from root is not a big deal and not providing any easily ready to use solution, I doubt that many will go extra mile to implement that on they own; given Cloudron limited resources and luck of advertising and hence focus to be security first platform, dovecot processes will remain to be running as root.
From the positive side, root owned processes are not opening any network port, so direct exploitation would be problematic.
Hope that would be of help.