Ciao,
prova a fare questa modifica, nel file doceboCore/addons/phpmailer/class.phpmailer.php

Cerca, verso la riga 472, il seguente blocco di codice:

if ($this->Sender != '' && strlen(ini_get('safe_mode'))< 1) {
   $old_from = ini_get('sendmail_from');
   ini_set('sendmail_from', $this->Sender);
   $params = sprintf("-oi -f %s", $this->Sender);
   if ($this->SingleTo === true && count($toArr) > 1) {
      foreach ($toArr as $key => $val) {
         $rt = @mail($val, $this->EncodeHeader($this->SecureHeader($this->Subject)), $body, $header, $params);
      }
   } else {
      $rt = @mail($to, $this->EncodeHeader($this->SecureHeader($this->Subject)), $body, $header, $params);
   }
} else {
   if ($this->SingleTo === true && count($toArr) > 1) {
      foreach ($toArr as $key => $val) {
         $rt = @mail($val, $this->EncodeHeader($this->SecureHeader($this->Subject)), $body, $header, $params);
      }
   } else {
      $rt = @mail($to, $this->EncodeHeader($this->SecureHeader($this->Subject)), $body, $header);
   }
}

e modificalo in:

if ($this->Sender != '' && strlen(ini_get('safe_mode'))< 1) {
   $old_from = ini_get('sendmail_from');
   ini_set('sendmail_from', $this->Sender);
   $params = sprintf("-oi -f %s", $this->Sender);
   if ($this->SingleTo === true && count($toArr) > 1) {
      foreach ($toArr as $key => $val) {
         $rt = @mail($val, $this->EncodeHeader($this->SecureHeader($this->Subject)), $body, $header, $params);
      }
   } else {
      $rt = @mail($to, $this->EncodeHeader($this->SecureHeader($this->Subject)), $body, $header, $params);
   }
} else {
   if ($this->SingleTo === true && count($toArr) > 1) {
      foreach ($toArr as $key => $val) {
        $rt = @mail($val, $this->EncodeHeader($this->SecureHeader($this->Subject)), $body, $header);
      }
   } else {
      $rt = @mail($to, $this->EncodeHeader($this->SecureHeader($this->Subject)), $body, $header);
   }
}

in pratica basta che togli il pezzo che ho messo in rosso.. fammi sapere ;)

ciao,
Giovanni.