Dovecot, maildir on Windows 2008r2 NFS share
Read the error message again. It says "euid is not dir owner" which means "the user with the id 5000 is not the owner of the directory".
Correct that. Authentication is not the problem here. Authorization is.
On the other hand: Don't do NFS that way with Dovecot! See http://wiki2.dovecot.org/NFS for details. And then NFS is no native service on Windows, which means another pitfall. I - personally - would never ever trust this kind of setup.