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I don't really know where to post feature ideas, but this seems the only viable option.
I was setting up a fallback MX server with Postfix and was struggling with preventing backscatter mail. I thought I found a good solution, but it turned out to be an illegal option.
Postfix has the ability to do recipient address verification. When postfix acts as a relay server, this prevents backscatter mail (bounces of messages because the server that is relayed to doesn't accept the user). Backscatter is usually caused by spam of course, because spam is sent to all kinds of users @example.com.
I had in mind to use recipient address verification to avoid that and then set "unverified_recipient_tempfail_action = permit". The idea behind this was:
- Prevent backscatter mail when the primary host is up because every address is verified first.
- Accept all mail when the primary host is down, so that incoming messages aren't deferred.
But permit is not a valid option for unverified_recipient_tempfail_action. Would it be an idea to implement this?
I know I can use permit_mx_backup and permit_mx_backup_networks, but I'd rather not have to maintain a list of networks on the fallback server, partly because I want to be a fallback server for servers that I don't maintain and of which I have no idea if the address changes.