I'm the author of the plugin WP-United. WP-United integrated phpBB forums into WordPress. A user of your plugin has reported a conflict with your function, validate_email() (in your wp_mail_smtp.php).
This function conflicts with an identically-named function in phpBB, validate_email(). Unfortunately this is not in WP-United code, so I can't change it; and I will be unable to influence phpBB to change it on their end, as it would break compatibility with phpBB plugins.
Please therefore could you rename your function to something else? In fact, I recommend namespacing all your functions with e.g. wpmailsmtp_function_name().
Hope this helps. The original report in question is here: http://wordpress.org/support/topic/issues-with-integrated-login-and-plugin-user-switching?replies=7#post-3947865