We use Postfix for some time now, and it's really a great relieve after having used qmail for years. However there's one issue I am unable to solve.
Next to the usual virtual mailboxes and aliases we also provide a fallback mail service to some customers that insist on having their own mail server in their office (usually connected to an ADSL line). Our server is a low-priority MX for these domains. When the customer's mail server or ADSL line is down we accept the mail for them, and deliver it to their server as soon as it is reachable again. To prevent backscatter I have set reject_unverified_recipient in the smtpd_recipient_restrictions. But when the customer's server is down, recipients can't be verified of course. So I set unverified_recipient_defer_code to 250, and mail is now accepted when the customer's server is down. Unfortunately some servers we relay for are quite slow on address verification sometimes, and then postfix defers (and accepts with code 250) the mail just before the customer's server answers that the address is invalid. This causes backscatter and I want to prevent that. I can imagine two solutions to solve this, but I cannot find how to do this: - Increase the timeout for address verification. It is something like 6 seconds by default, but I can't find a setting for it. - Set unverified_recipient_defer_code to the default 450 (which of course is much better anyway), and return 250 only when the customer's server is completely unreachable. Can this be done? Of course it would be better if we didn't use this fallback construction at all, but some customers just love it. Thanks a lot Mark
