Roller Network Help - phpBB SMTP Authorization
The following example is provided for informational purposes only. We do not provide technical support for mail clients or officially support any specific mail client.
phpBB incorrectly uses the SMTP server address as the parameter for EHLO, which effectively means that it's trying to HELO to the server with the server's own hostname. There's a mod for phpBB that fixes this:
http://www.phpbb.com/phpBB/viewtopic.php?t=280850
Or, you alter the code yourself. In the file includes/smtp.php look for the line:
fputs($socket, "EHLO " . $board_config['smtp_host'] . "\r\n");
Change it to:
fputs($socket, "EHLO " . $board_config['server_name'] . "\r\n");