Thanks again for the great product, I cannot say it enough. I like the Nextcloud integration as well, we were even able to easily phase out Dropbox-like services.
We are running on Linode, and in order to resolve the “Linode issue” they assigned us a /64 range. I enabled the range on our system and configured this as our “Public IPv6” address. I successfully reconfigured mailinabox.
However, even with this change, some remote mailservers still block us with a new error, according to them the PRIVATE IP is not allowed to send mail for that domain (SPF record error). I assumed (wrongly?) the private IP is not used for outgoing connections. How does this work?
I tried to configure the private IP address to this range as well, but was unable to. It came back with the original private IP.