Hi,
at
http://alcazaba.unex.es/~mperaya/pgina/contrib/ you can download and try a modification of classic email auth plugin that allows to include many pop3 or imap auth sources at a time.
This plugin trust auth when any (normally the first correct auth) of the remote servers validates the username and password of the user.
We have worked some time recompiling many versions of the original email plugin for this purpose, but finally we have found the time to modify the plugin to allow this more flexible way of work.
Hope it may be usefull for anybody.