The next version will have a POP/IMAP option. This will be useful to those in chroot environments, or those on multiple servers, or using external mail servers.
I have it working, but I need some test volunteers. If you have a POP account for the subscriber_notify account, send me the POP access info by PM. (The worst I can do is delete a couple of bounced emails!)
I can't install Exim on my system, since I never used it, and would be time consuming for me.