I’d got multiple SMTP transports on my postfix server, but one of these send’s newsletters,
As we already seen this in real life of an mailadmin some servers got rate-limit’s setup, so only a specific
amount of mail is accepted in a specific timeframe.
To get these masses sent to these destinations in a adequate timeframe we need a fallback relay.
Due to the configuration of the destination server these will accept mails from other client IP’s than
the rate-limited server is and they would send an 450 temporary error.
Long speech short sin.
Here as always my config snippet from postfix master.cf (somewhere at line 40):