[exim] help with return_path

I posted this query a minute ago, when is was not a list-member.

I am using latest Debain unstable, exim4 4.34
I am on a laptop, attempting to send email via smtp_smarthost.

With the default Debian config, 'return_path' is filled in
via a DebConf macro expansion ( which i don't yet understand )

Debconf give me this in /var/lib/exim4/config.autogenerated:
( broken up for readability )

return_path =
${if match_domain{$sender_address_domain}{+local_domains}
{
${sender_address_local_part}@DCreadhost}
${if match_domain{$sender_address_domain}{macbob}
{
${sender_address_local_part}@DCreadhost}
fail}
}

Which produces this error when i try to send a message:

2005-01-17 10:16:16 1CqYsr-0002YF-4G == cliffw@??? R=smarthost T=remote_smtp_smarthost defer (-1): Failed to expand return path "${if match_domain{$sender_address_domain}{+local_domains}{${sender_address_local_part}@macbob}${if match_domain{$sender_address_domain}{easystreet.com}{${sender_address_local_part}@macbob}fail}}": syntax error in "if" item - "fail" expected
--------------
After some trial-and-error, i changed exim4.conf.template to this:
return_path = \
${if match_domain{$sender_address_domain}{easystreet.com}\
{\
${sender_address_local_part}@DCreadhost}\
fail}\

--------------
This does not produce errors, but i don't think it's doing the Right Thing, either.

Can anyone help me out with this syntax? I'm very new to this.
cliffw