I've just set up exim on my ubuntu computer. At the moment it will only accept email for accounts that exist on that computer but I would like it to accept all email (just because I'm interested). Unfortunately there seem to be a million exim related config files, and I'm not having much success finding anything on google.
Is there an introduction to exim for complete beginners?
Thanks.
There's a mailing list at http://www.exim.org/maillist.html. The problem you will face as an Ubuntu user is that there's always been a slight tension between Debian packagers/users and the main Exim user base because Debian chose to heavily customize their configuration. Their reasons for customizing it are sound, but it results in Debian users showing up on the main mailing list asking questions using terms that aren't recognizable to non-Debian users. Debian runs its own exim-dedicated help list (I don't have the address handy, but it's in the distro docs). Unfortunately this ends up causing you a problem because Ubuntu adopted all these packages from Debian, but doesn't support them in the same way as Debian does, and Debian packagers seem to feel put upon to be asked to support these Ubuntu users.
So, Ubuntu user goes to main Exim list and is told to ask their packager for help. So they go to the Debian lists and ask for help and may or may not be helped.
Now, to answer your original question, there are a ton of ways to do what you ask, and probably the best way for you is going to be specific to the Debian/Ubuntu configurations. However, to get you started, you could add something like this to your routers:
catchall:
driver = redirect
domains = +local_domains
data = [email protected]
If you place that after your general alias/local delivery routers and before any forced-failure routers, that will redirect all mail to any unhandled local_part at any domain in local_domains to [email protected].
local_domain is a domain list defined in the standard exim config file. If you don't have it or an equivalent, you can replace it with a colon-delimited list of local domains, like "example.com:example.net:example.foo"
One of the reasons it's hard to get up to speed with Exim is that you can literally do anything with it (literally, someone on the list proved the expansion syntax is turing complete a few years ago, IIRC). So, for instance, you could use the above framework to look the domains up out of a file, to apply regular expressions against the local_parts to catch, save the mail to a file instead of redirecting to an address, put it in front of the routers and use "unseen" to save copies of all mail, etc. If you really want to administer an Exim install, I strongly recommend reading the documentation from cover to cover, it's really, really good once you get a toe hold.
Good luck!