Because there were always script violations. I modified the following lines in the 
Email.lng. 
 Search: 
<a href="{yabb scripturl}?action=activate&username={yabb cryptusername}&activationkey={yabb activationcode}">{yabb scripturl}?action=activate&username={yabb cryptusername}&activationkey={yabb activationcode}</a> 
 Replace: 
{yabb scripturl}?action=activate&username={yabb cryptusername}&activationkey={yabb activationcode} 
  Search: 
You may change your password after you login by clicking on the User CP tab at the top of the {yabb mbname} main page: <a href="{yabb scripturl}">{yabb scripturl}</a> and then click the Profile tab, click on Edit Profile (you will need to enter your password as a security measure). 
 Replace: 
You may change your password after you login by clicking on the User CP tab at the top of the {yabb mbname} main page: {yabb scripturl} and then click the Profile tab, click on Edit Profile (you will need to enter your password as a security measure). 
  ... and so forth 
 In short:
 Removes all html code, it is absolutely not needed. 
 If you can't figure it out, please contact me.   
Mailer.pm  Search:
     elsif ( $mailtype == 1 ) {
         $smtp_to      = $to;
         $smtp_from    = $from;
         $smtp_message = qq~<pre $pre>$message</pre>~;
         $smtp_subject = $subject;
         $smtp_charset = $charsetheader;
         require Sources::Smtp;
         use_smtp();
     } 
  Replace: 
    elsif ( $mailtype == 1 ) {
         $smtp_to      = $to;
         $smtp_from    = $from;
         $smtp_message = qq~$message~;
         $smtp_subject = $subject;
         $smtp_charset = $charsetheader;
         require Sources::Smtp;
         use_smtp();
     } 
  Search: 
        eval q^
             $smtp->mail($from);
             foreach (split /, /sm, $to) { $smtp->to($_); }
             $smtp->data();
             $smtp->datasend("To: $toheader\r\n");
             $smtp->datasend("From: $fromheader\r\n");
             $smtp->datasend("X-Mailer: YaBB Net::SMTP\r\n");
             $smtp->datasend("Subject: $subject\r\n");
             $smtp->datasend("MIME-Version: 1.0\r\n$cte\nContent-Type: text/html\; charset=$charsetheader\r\n");
             $smtp->datasend("\r\n");
             $smtp->datasend("<pre $pre>$message</pre>");
             $smtp->dataend();
             $smtp->quit();
         ^;
         if ($EVAL_ERROR) {
             fatal_error( 'net_fatal',
                 "$error_txt{'error_verbose'}: $EVAL_ERROR" );
         }
         return 1;
     } 
  Replace: 
        eval q^
             $smtp->mail($from);
             foreach (split /, /sm, $to) { $smtp->to($_); }
             $smtp->data();
             $smtp->datasend("To: $toheader\r\n");
             $smtp->datasend("From: $fromheader\r\n");
             $smtp->datasend("X-Mailer: YaBB Net::SMTP\r\n");
             $smtp->datasend("Subject: $subject\r\n");
             $smtp->datasend("MIME-Version: 1.0\r\n$cte\r\nContent-Type: text/html\; charset=$charsetheader\r\n");
             $smtp->datasend("\r\n");
             $smtp->datasend("$message");
             $smtp->dataend();
             $smtp->quit();
         ^;
         if ($EVAL_ERROR) {
             fatal_error( 'net_fatal',
                 "$error_txt{'error_verbose'}: $EVAL_ERROR" );
         }
         return 1;
     }