@girish Yes, I no longer got the Gmail issues when I created my own system service to implement the iptables rule. So it seemed to do the trick.
The IP tables rule to add is really just this: iptables -t nat -I POSTROUTING -s 172.18.0.0/16 -o enp3s0f0 -j SNAT --to-source {FLOATING_IP}
Where FLOATING_IP is really just replaced with whatever the recognized IP address is that’s used in the DNS records for the MX record. I supposed it could be further improved to only be applicable to the mail container rather than all Docker traffic. And of course the interface would have to be dynamic too.
I guess an alternative is for me to create additional MX records with the other IP addresses but then it’s manually done and prone to mistakes/issues.
In my opinion, I think we really need an option to select the outbound IP interface using Cloudron for the mail component, in order to avoid the Gmail issues (and any other provider who will use FCrDNS for verifying or rejecting emails in the future). I recognize this may not be a common concern as most people probably only have the one IP address of each type and so the DNS records if setup automatically by Cloudron will use both the IPv4 and IPv6 address, but for those of us who use a floating/failover IP addresses that we want to be the one true IP address being used, this becomes an issue without that workaround in place.
