I'm not a fan of catchall mailboxes, they are a legacy of POP mail systems and getting Exchange to do it has always been a bodge. However the need arose recently with a client we migrated to SBS2008 so I decided to see what the options are for doing it in Exchange 2007.

A quick web search will probably turn up several sites explaining how to use transport rules to achieve a catchall mailbox, and they do make it very easy. Unfortunately there is one major drawback, the required transport rule can only be used on an Edge Transport server, not a Hub Transport, so if you have a single server Exchange 2007 deployment that's not an option.

Fortunately a man called Wilbert has come to our rescue and created an agent which provides the catchall functionality, you can download it from here: http://catchallagent.codeplex.com/ . View the "README.txt" file for instructions on how to install and configure the agent, its very simple and just requires a couple of lines in the Exchange Management Shell.

 There is one thing which may catch you out if you're using SBS2008 - if you get an "invalid agent configuration" error referring to the agent.config file when you try to install the catchall agent. It recommends running a "restoreagentdefaults" script but if you try that you just get more errors. I eventually found that its simply a case of access permissions - the admin account SBS prompts you to create when you set it up doesn't have write/modify access to the agents.config file. All you have to do is browse to the location of the file and give your admin account "Full control" rights then try the agent install again and it should work fine. 

Update

You can now see my detailed guide explaining how to set up an Exchange 2007 catchall mailbox on the Petri IT Knowledgebase website:

http://www.petri.co.il/setting-up-a-catchall-mailbox-in-exchange-2007.htm