On Friday 17 June 2005 10:14, Casey Allen Shobe wrote:

By making your backup MX aware of all the valid recipients for all the domains you are accepting on that MX. This is usually done with LDAP, so that you can have a local replica that is always up to date (minus network outages, which are rare enough these days that this goes within the 0.01% downtime you're likely going to have anyway).

I use postfix, which has a very nice option called relay_recipient_maps that does just this. It makes a lot of sense to do this on _every_ of your secondary MX servers. It also makes greylisting worthwile, and it eliminates more than 90% of all spam and virii before they even hit the av/spam filters.

You can't really tell. You're likely not on any public blacklists, but that doesn't mean that somebody didn't blacklist you when they got enough of your bounces to their MX, because spammer used their domain when forging source addresses.

Doubting the security of a system is the first step towards securing it. Making sure you don't accept mail for nonexistent local users is very polite towards the internet, but of course it potentially reveals valid local recipients' addresses throught dictionary attacks. The choice is yours, but I strongly urge you to consider the internet-friendly option. That way, the spam problem is likely going to be reduced instead of aggravated. Most of bigger systems do this because the huge amount of junk they could have sent as bounces would have been disastrous for smaller systems.