Hi,
If you're talking about custom product fields, each one will be in its own column with one cell per product, so it should not be a problem.
If you're talking aobut custom item fields, each one is in a p tag.
And by default there is no CSS on p tags in the emails so you should already have one per line in the emails, unless you added custom CSS to change that.
In that case, you can add such CSS to avoid that:
table p[ display: block !important; }
You could also add a <br/> tag to the line:
$t .= '<p>'.$fieldsClass->getFieldName($field).': '.$fieldsClass->show($field,$item->$namekey,'user_email').'</p>';
in the preload area of the email.