We're exploring the possibility of using smtp_fallback_relay as a way to offload re-delivery attempts of deferred mails when we send our weekly newsletter to 700K+ recipients.
>From the docs at http://www.postfix.org/postconf.5.html#smtp_fallback_relay, here's how I understand this would work: 1) mailer.sendingdomain.com sends the original message. Let's assume each message has a Return-Path of "variableaddr...@bounce.sendingdomain.com" and that bounce.sendingdomain.com is a virtual mail host of mailer.sendingdomain.com with a catchall account to get the bounces (this is how we're currently set up). 2) On the first delivery attempt, smtp.receivingdomain.com replies with a 4xx message saying "I can't deliver this right now, but try again later" 3) fallbackmailer.sendingdomain.com is set up as the smtp_fallback_relay, so it handles subsequent re-delivery attempts, freeing up more resources on mailer.sendingdomain.com to handle first attempts at deliveries and incoming mail. 4) Successful re-deliveries get logged in fallback's /var/log/maillog 5) Hard bounces (whether 5xx messages from smtp.receivingdomain.com or timeouts where fallbackmailer.sendingdomain.com gives up) get delivered to the catchall on bounce.sendingdomain.com (where we can process them later). Two questions: 1) Is that an accurate description of what would/could happen with smtp_fallback_relay? 2) Am I accurate in assuming that smtp.receivingdomain.com will see delivery attempts from both IP addresses for mailer.sendingdomain.com and fallbackmailer.sendingdomain.com, and therefore I will need to manage the Sender Reputations of both IPs, make sure they are both included in FBLs, absent from blacklists, etc.? 3) Won't this cause an issue with DKIM validation? If the original message was signed by mailer.sendingdomain.com, won't it fail validation on the receiving end since the fallback relay has a different hostname? If so, any possible solutions to this? Thanks, Steve
