How do I render the absolute URL of a referenced entity in a simplenews newsletter?
external and absolute are different:

external is an url without internal route. You can't change this property. This is set when you create an url.

absolute is an option, which you can set at any time in the url object:

$url-&gt;setAbsolute();


Or set to false:

$url-&gt;setAbsolute(FALSE);


And this will change how the url is generated absolute or relative.