Edit the file mailings.acajoom.html.php from the directory "<Joomla>/administrator/components/com_acajoom" and from line 141 to 148, you will see the parameter $show['index'] several times :

   if ( function_exists('sefRelToAbs') AND $GLOBALS[ACA.'use_sef'] ) {
    $link = sefRelToAbs($show['index'].'.php?option=com_acajoom&act=' . $action . '&task=view&listid='.$mailing->list_id.'&mailingid=' .$mailing->id . $item );
  } else {
    $link = $show['index'].'.php?option=com_acajoom&act=' . $action . '&task=view&listid='.$mailing->list_id.'&mailingid=' .$mailing->id . $item;
  }
} else {
  $link = $show['index'].'.php?option=com_acajoom&act=' . $action . '&task=view&listid='.$mailing->list_id.'&mailingid=' .$mailing->id .$item;
}

If you replace each $show['index'] by 'index2.php', that will do the job : 

   if ( function_exists('sefRelToAbs') AND $GLOBALS[ACA.'use_sef'] ) {
    $link = sefRelToAbs('index2.php?option=com_acajoom&act=' . $action . '&task=view&listid='.$mailing->list_id.'&mailingid=' .$mailing->id . $item );
  } else {
    $link = 'index2.php?option=com_acajoom&act=' . $action . '&task=view&listid='.$mailing->list_id.'&mailingid=' .$mailing->id . $item;
  }
} else {
  $link = 'index2.php?option=com_acajoom&act=' . $action . '&task=view&listid='.$mailing->list_id.'&mailingid=' .$mailing->id .$item;
}

After editing these lines, go to the line 163 :

<td align="left"><a href="<?php echo $link;  ?>" >

And replace it by : 

• If you want to open in a new page :

<td align="left"><a href="<?php echo $link;  ?>" target="_blank" >

• If you want to open in a popup :

<td align="left"><a href="#" onClick="window.open('<?php echo $link;  ?>', 'Detail', 'toolbar=0,scrollbars=1,location=0,statusbar=1,menubar=0,resizable=1,width=600,height=500');" >

Save the file.