[Exim] exim and imap without using /etc/passwd

I'm trying to set up email for a few hundred users, but they don't need
shell accounts and I'd rather not use /etc/passwd to manage their
accounts. The "features" section of the exim manual says I can set
things up without creating real accounts, but I haven't found a simple
explanation of how to do this.

A plain text file with each line containing username and password would
be perfect, but it's not clear how to get exim to accept mail to people
listed in there (SMTP auth doesn't seem to be what I need). A database
would also be fine if that's simpler to set up.

Does someone have a situation similar to mine who would be willing to
share the relevant parts of their configuration? Or if this is
explained somewhere, a pointer would also be fine. Info on how to set
up the IMAP end of it would also be appreciated.