[Pkg-exim4-users] Regression in testing? Local mail passed to smarthost, ignoring aliases

[Michael Below]

Hi,

I have tried to configure Exim on two similar systems: Both are
"satellites" without local mail delivery, mainly I want the mail sent
to root to be forwarded to an external address. Initially, I stumbled
onto the issue that mail to just "root" would be sent to to the
smarthost, which did not know "root@<myexternaldomain>". I was able to
solve this for one system running Debian stable, but not for the other
system running Debian testing, while following exactly the same steps.
I would like to understand what I need to do differently on testing.

I set up the smarthost according to
https://wiki.debian.org/Exim#Communicating_with_a_smarthost

Then I tried the solution as proposed in 
https://unix.stackexchange.com/questions/235112/exim-email-to-root-ignores-etc-aliases-and-root-forward-always-used-qualif

I.e. I set up the system mail name according to "hostname -f" (which
returns just a simple name, no FQDN), and that mail should also be
accepted for <myexternaldomain>, and that <myexternaldomain> should be
used as a visible domain name. Then I set an alias for root in
/etc/aliases pointing to e.g. "mail at separatedomain.com".

For stable it works, I get mail sent to "root at onehostname" forwarded to
"mail at separatedomain.com". For the system running testing it does not
work, I get error messages from the smarthost that the smarthost had a
"name service error" regarding the mail address "root at otherhostname".It
seems like the exim4 on Debian testing does not accept responsibility
for delivery to it's own hostname, while the Debaian stable system
attempts delivery and follows the alias.

Why does exim4 on stable use the alias, and exim4 on testing ignores
the alias?

Thanks for any pointers...

Cheers
Michael