WordPress.org

Reliable and flexible plugin. My notice if someone want to use it at frontend. There is an error when you try to view specified message, because inbox and outbox collision.
Solution:
1) Open plugin folder with includes:
\wp-content\plugins\private-messages-for-wordpress\inc
2) Open inbox-page.php and outbox-page.php in text editor. Find in each page the line which follows "// if view message" comment (usually line 10)
replace it with
if (isset($_GET['page']) && 'rwpm_outbox'== $_GET['page'] && isset($_GET['action']) && 'view' == $_GET['action'] && !empty($_GET['id'])) {
for outbox function, for inbox it should be 'rwpm_inbox' page in condition.

This is a cool "internal" messaging system. I use it on a site where students and mentors and instructors need to communicate with each other.