I have host for running BackupPC (name it BPC). It is running nothing else and not part of ISPConfig cluster. I have exim4 as mail server, since it is simpler than Postfix. Should it work if I configure BPC to use as smart host my ISPConfig e-mail server MAIL? Then add on MAIL the IP-address of BPC to mynetworks in postfix configuration. MAIL is installed with ISPConfig autoinstall on Debian 12, and has both IPv4 and IPv6 addresses. Should auto-installed ISPConfig work for as smarthost e-mails with both IPv4 and IPv6 equally well? I'm asking because I already tried doing the above and created a disaster. After restart of postfix, this lead to situation, where e-mails started failing. Eventually I figured all e-mails from anywhere were rejected if they were going to mailboxes on MAIL. I did not discover this error situation right away, since I tested smarthosting and that did work since the recipient e-mail address was not on MAIL. On hosts trying to send to MAIL messages remained in mailq with error message "Temporary lookup failure (in reply to RCPT TO command))". Just pottering about trying to fix it, I removed the IPv6 address from mynetworks. That fixed the issue. Reading the docs [http://www.postfix.org/postconf.5.html#mynetworks] I saw that in mynetworks IPv6 addresses must be written in brackets. I guess this caused the issue, I had copypasted the address and not added brackets. But I do not understand why syntax error in mynetworks makes host reject mail it should handle.
That#s how I would configure it as well. Most likely, Postfix does not recognize the IPv6 address then, or postfix sees the whole mynetworks as invalid then.
