<***@gmail.com>: host gmail-smtp-in.l.google.com[18.104.22.168] said: 550-5.7.1 Unauthenticated email from yahoo.com is not accepted due to domain's 550-5.7.1 DMARC policy. Please contact administrator of yahoo.com domain if 550-5.7.1 this was a legitimate mail. Please visit 550-5.7.1 http://support.google.com/mail/answer/2451690 to learn about DMARC 550 5.7.1 initiative. 19si20642687wjx.29 - gsmtp (in reply to end of DATA command)
A better option in my opinion would be to set the reply-to header with the customer email address and leave the from field with the default address.
Something along these lines (in Misc.php):
$mail->send( XenForo_Application::get('options')->contactEmailAddress, '', array( 'Reply-To' => $user['email'] ), XenForo_Application::get('options')->defaultEmailAddress, XenForo_Application::get('options')->boardTitle );