Might be useful for someone. This is a quick and dirty script to get all Hestia mail domain names for all Hestia users into the Proxmox Mail Gateway’s Relay Domains list, so all incoming emails that comes through the PMG can be then relayed to Hestia - this will give you granular filter and incoming mail cluster support.
https://blog.campodoro.org/?p=2380
Ps. If I cannot link blog-posts here, I can also copy-paste it. Cheers!