I think that sometimes we forget it, too. :)

Thanks for the tip. I will try it. I forget that the config files are 
php files too,  so they allow this degree of flexibility.

Instead of using hooks, or the preferred method, and instead of 
defining all 3 backends in backends.php, use your hooks code right in 
backends.php to figure out which backend the user belongs to, then 
define only that one.

Ok. Sorry for that. I use to work on stable releases to get the 
patches working on our production system as fast as possible.

I'll modify it using a CVS checkout if you finally find this patch useful.
Well, getBackend() in lib/Ingo.php selects the backend wich its 
'preferred' field equals $_SERVER['SERVER_NAME'] or 
$_SERVER['HTTP_HOST']

In our University we have only one URL to access the webmail system 
but three different IMAPservers wich need different backends (procmail 
and sieve, the 3rd server is Exchange), so the unpatched system always 
uses the same backend.

Currently, I have no idea on how to achieve this using backends.php

Sorry if I was unclear - new features should always be against HEAD, 
not a release version (see http://horde.org/source/contribute.php), so 
that's what I was applying it against.

What about my question on why this is necessary?

I've just downloaded back the patch and a fresh copy of Ingo-h3-1.1.1 
from ftp.horde.org and it applies without errors to both files.

The patch to Ingo.php does not apply for me.

Also, is this really something that can't be done inside the 
backends.php config file?

Here's the updated version of the patch with the requested changes.

I've opened an enhancement ticket  for the passwd module to implement   
a similar functionality (see ticket #3821)

I'll update it accordingly when we close all the pending issues with 
this ticket

Sure.
Here you have an example of the custom hook. I need a little more time 
to address the rest of the issues. I'll upload it tomorrow.

Can you update your patch to use Horde::callHook() instead, use a 
different hook name (_ingo_hook_backend_select), and provide an 
example for horde/config/hooks.php.dist?

Looks good.

The attached file contains two patches against ingo-h3-1.1

conf.xml.patch for config/conf.xml
Ingo.php.patch for lib/Ingo.php

They add suport for one custom hook for backend Selection 
(_ingo_hook_selectBackend)