[exim] set return_path in config gives error

Hi everyone!

I want to set the return path to $sender_address, so I checked the
documentation here:

https://www.exim.org/exim-html-current/doc/html/spec_html/ch-some_common_configuration_settings.html


And tried applying that to my smarthost config:

smart_route:
driver = manualroute
domains = !+local_domains
transport = remote_smtp
return_path = $sender_address
#return_path = ${if{$return_path}{$sender_address}fail}}
route_list = * my.internal.ip.address

but I get an error "option "return_path" unknown". What am I doing
wrong?

thanks in advance! Andy.