When the smtp setting is Off in config.inc.php, OJS will use PHP's mail() function.
If you're using your ISP's mail server (which should be the case when using PHP's mail() function), it shouldn't matter whether or not users are behind firewalls.
Are all users failing to receive OJS emails? Or do some work? Have you checked to see if they're being caught by a spam filter?
Open Journal Systems Team
Don't miss the First International PKP Scholarly Publishing Conference
July 11 - 13, 2007, Vancouver, BC, Canada