https://www.postfix.org/STANDARD_CONFIGURATION_README.html https://www.postfix.org/STANDARD_CONFIGURATION_README.html Look at the bit about "backup MX for a re

https://www.postfix.org/STANDARD_CONFIGURATION_README.html https://www.postfix.org/STANDARD_CONFIGURATION_README.html Look at the bit about "backup MX for a remote site". The specific thing you are asking for, however, isn't a backup MX so much as a second delivery to a local mailspool, which is also doable now that you know what it's called.