To solve this, go to include/email.php:

line 78:

delete this line:

$headers = 'From: '.$from."\r\n".'Date: '.date('r')."\r\n".'MIME-Version: 1.0'."\r\n".'Content-transfer-encoding: 8bit'."\r\n".'Content-type: text/plain; charset='.$lang_common['lang_encoding']."\r\n".'X-Mailer: PunBB Mailer';

and replace with this:

        $replyto = $pun_config['o_webmaster_email'];

    if ($from != $pun_config['o_webmaster_email']) {
        $replyto = $from;
    } 
            
    $headers = 'From: '.$from."\r\n".'Date: '.date('r')."\r\n".'MIME-Version: 1.0'."\r\n".'Content-transfer-encoding: 8bit'."\r\n".'Content-type: text/plain; charset='.$lang_common['lang_encoding']."\r\n".'Reply-To: '.$replyto."\r\n".'X-Mailer: PunBB Mailer';

note the first line makes that all automatic messages they now have an automatic reply to your webmaster address. If you don't want replies in your main email address, then the best thing is creating a new e-mail account in your server, in example rubbish@mydomain.com. In this way,  those e-mails will be received in that account which must be cleaned periodically. In this case,  you will need to replace the first line of this patch with your new e-mail address for the unwanted replies:

    $replyto = 'rubbish@mydomain.com';

that's all. Rest of messages between users inside the forum will work normally.

--

What I have is several users of Yahoo and Hotmail, who cannot receive e-mails, both from registration process or  from the forgotten passwords.

I have check these messages truly are leaving from the server. Therefore there is something in the email headers that Yahoo and Hotmail don't like and probably they are blocked like spam. 

Specially, the e-mail headers lacks of a Reply to: field, and also they contain a
Received: from nobody by host.mydomain.com with local ....

maybe the sum of these two causes that blocking. I'm looking how replacing this issue.

Somebody know about that?

return preg_match('/^(([^<>()[\]\\.,;:\s@"\']+(\.[^<>()[\]\\.,;:\s@"\']+)*)|("[^"\']+"))@((\[\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}\])|(([a-zA-Z\d\-]+\.)+[a-zA-Z]{2,}))$/', $email);

but i don't know what to write instead.
Any helps pleas!
Regards