let’s say – for arguments sake – you want to add a dotted line after every item in your plaintext emails. you could use a filter like this in your theme’s functions.php file

/**add line and linebreak after each item in plaintext emails***/
add_filter('wppizza_filter_plaintextemail_item_markup', 'my_plaintext_email_item_lines');
function my_plaintext_email_item_lines($itemMarkup){
$itemMarkup['add_dotted_line']='-----------------------------------------------------'.PHP_EOL;
return $itemMarkup;
}