It's been awhile since I messed with this, but as I recall, this setting will do just that:
1. Open the Replication Settings of the mail.box.
2. Click on the Space Savers tab.
3. Enable the setting to Remove documents not modified in the last X days and set it to the value you want.

Be sure to do this to the template too, so if a new mail.box is created, it gets the same setting.

This is rather obscure, but I think it does just what you want. Yes, it would be convenient to have a setting on the configuration document, but not really necessary.

@David, while that is certainly a good workaround, I think that there is some merit to Mark's idea. I am promoting this; there should be a better way.

@David: What triggers the purging? Is it updall?

Ooh, now we're playing "Stump the chump." I know it happens the first time you open a database in awhile (local databases at least) and yes, since updall is the process to clean up deletion stubs, I would assume it also handles deleting documents per this setting.

Greg, I don't disagree with you. Just given how big the form is already and given so many other things we would all like to see that have no workaround, I defer for now.

I haven't seen dead mail in the mail.box since I upgraded to version 6, and implemented the incoming mail checks, so I think this is a non-issue. As David suggests, there's also a functional solution for this already, so I'm demoting this.

@Peter, that is fine if your environment is limited to incoming mail. I run a public web application which requires users to enter email addresses, which must then be confirmed by the standard mail-out method I'm sure you're familiar with. In my case, I have thousands of new registrations weekly, many mistyped, invalid etc. So they often end up returned and become dead mail.

@David, I did some testing, and it appears the purge occurs automatically when a database is accessed (ie via a UI or programmatically). I saw it on the server log "purging old documents from admin4.nsf" and there was definitely no updall running. I don't mind your solution by the way - I have enabled it.

@Mark, If your aplication is sending mail with invalid addresses, it should also have a valid return address. The return address should be in an application that could properly handle the return. It's a bug in your application, not a fault in the infrastructure.

Peter, maybe I should pay you to come help me :)

@Mark, It's not hard to do. All you really need is a Mail-In-Database document in the Domino Directory, and preferrably a view for incoming mail, and an agent to process the mail. You could even create a separate database/application to use as a black hole. One agent to delete all incoming mail, if you're not interested in checking them.

This is easy stuff, nothing to it. You difinately don't need me for it... :-)

A whole new feature just for managing dead mail isn't the way. Enhancements to server-based mail rules to prevent mail from going dead in mail.box in the first place is something I would support.