Please note: This is a work in progress. I do hope that this will eventually expand into a full-blown how-to, but I will probably need some assistance with it. I will also putting notes down here as I work on a complete backup-mail server solution using ISPConfig. Feel free to make comments and questions as I work on it. Thanks. The situation I've been working on setting up a backup main server for my domains for some amount of time. There are several guides out there that show you how to set one up with postfix, but they have one problem: All of the ones I've read will setup a relay server that will simply attempt to forward any mail to the entire domain to the primary server. What then happens in this case is the primary server will reject any email addresses not in its local users table, and the backup mail server will be forced to bounce the message back to the from email address. This can cause a potential problem, though; some unscrupulous person can use the backup server in a "backscatter" attack, where he will send many emails to the backup server to non-existent users that are in its domain and cause the backup email server to bounce messages back tto the forged "from:" addresses, annoying end victims and painting your backup server as a culprit in this "spam" and possibly blacklisted for being in accordance with the RFCs! The Solution One possibly solution is to simply go sod with the RFCs and prevent the backup mail server from bouncing emails, but this is a bit of a hack, does not comply with the RFC standard, and is inconsiderate to legitamite users that may have mistyped an email and should know that the message was not successfully delivered. The better way is to maintain a list of relayable recipent addresses on the backup mail server that will be syncronized with the primary mail backup server's, virtual users list. The How-To This applies to the current version of ISPConfig (version 2.2.6). This section is incomplete, but I will include an example to help with fleshing out this idea An example setup Let's say we have two servers with ISPConfig setup on both running with postfix as the MTA. Let's call them server1.maindomain.tld and server2.maindomain.tld. Let's say we have a domain hosted on server1,:called www.hosteddomain.tld, with server users with the addresses firstname.lastname@example.org and email@example.com. We want to setup server2.maindomain.tld as a backup mail server for hosteddomain.tld, so our DNS records will have the following information for this domain: Priority: 10, Host: @, Goes to: server1.maindomain.tld Priority: 20, Host: @, Goes to: server2.maindomain.tld On server2.maindomain.tld, we need to modify /etc/postfix/main.cf by adding the following two lines: Code: relay_domains = hash:/etc/postfix/relay_domains relay_recipient_maps = hash:/etc/postfix/relay_recipients Then create /etc/postfix/relay_domains with the following text: Code: hosteddomain.tld relay Then run the following commands: Code: postmap /etc/postfix/relay_domains Next, create /etc/postfix/relay_recipients with the following text: Code: firstname.lastname@example.org relay email@example.com relay Then run: Code: postmap /etc/postfix/relay_recipients The "relay" column can actually be anything, like "x" or something. It's just that postmap requires there be two actual columns (it seems). server2.maindomain.tld should now be acting as a backup mail server for the hosteddomain.tld domain (according to most such guides on the net and my own testing). Now, whenever a new user is added in the primary server, a new entry in "relay_recipents" will have to be added and postmap /etc/postfix/relay_recipients run to add the new user in the backup server's relay list. My observations to flesh out an auto-syncronizing system between Primary mail server and the Backup mail server(s) On the primary mail server, I've observed a file that would make this much simpler. This is /etc/postfix/virtualusertable. It seems simply copying this file to the backup mail server as /etc/postfix/relay_recipents with the domains setup in relay_domains might be sufficient to syncronize the list and make the backup server reject any invalid "to:" addresses without having to have itself bounce a message. The nice thing is virtualusertable also includes any aliases of users on the primary system. What I plan on doing is creating a script that will do the following on the primary server: 1) Make a copy of the virtualusertable file in a location where the backup mail server can retrieve it, preferably http locked with a user and password login, everytime this file is modified or running as a cron job. On the backup server, I'm looking into a script that will do the following in a cronjob: 1) Retrieve the file from the primary server (probably using wget). If the retrieve failed, cancel the rest of the script. If it retrieves it, save it temporarily as "retrievedvirtualusertable". 2) Detect any changes. 3) Parse through retrievedvirtualusertable and create a file called retrieveddomains, where it would only have all of the domains list only once. 4) Match retrieveddomains with the backup server's local-host-names, save anything in retrieveddomains but not in local-host-names in /etc/postfix/relay_domains. 5) Run "postmap /etc/postfix/relay_domains" 6) Match retrievedvirtualusertable with the backup server's virtualusertable, save anything in retrievedvirtualusertable but not in virtualusertable in /etc/postfix/relay_recipient. 6) Run "postmap /etc/postfix/relay_recipient". Well, this is where I'm at so far. Hopefully, I'll have time in the next week to create these scripts to do that. If you have any comments, feel free to reply in this thread. Thanks.