[Exim] security = unprivileged

I have just reconfigured exim to run with the security = unprivileged
option as described here:

http://www.exim.org/exim-html-3.30/doc/html/spec_55.html#SEC922

This is working fine, with exim owning the mailboxes.

None of my local_parts have system accounts.

Now I find that the pop3 daemon I use, popa3d, will not allow delivery
without system accounts.

Can you recommend a pop3 server to use for this configuration?

There are a number which say that they do not require system accounts,
but I can't find any documentation for any of them which describes how
to set the uid for the pop3 server after authentication without an
account.

I have been looking at:
cyrus
cucipop
teapop
courier

I only manage mail in the hundreds per day so high performance is not a
requirement. I am much more interested in security then performance at
any rate.

     Thanks,
          Michael