I am using Hestia as a control panel. I am using the default letsencrypt.org capability in Hestia and copying the DNS records to Cloudflare for SSL. It's not clear to me if I need to install a certificate for each domain separately for e-mail as it seems my VPS domain is vps##.heliohost.us is referenced when I generate an outbound e-mail programmatically from one of my domains. I can play with it, but if there are already instructions that would be great.
RE: Gmail: I got the impression that with so many hosts in your network google is more likely to block emails sourced from heliohost IPs or domains. If that is true and there is a higher likelhood of blocking than somewhere else, that is a problem for me. I have not noticed the issues you mentioned with Gmail undeliverability etc. I have not configured reverse DNS records so perhaps that is the root of my problem? Again, if documented somewhere let me know or I can research myself. I think it would be helpful for others to have best-practices instructions here to help people with this issue in the future.