[exim] "ultimately generated from"

Hi.

I'm using /etc/exim/domain-aliases.conf:

*.foo.com foo-mailman@???

To send all mail to *.foo.com to the foo-mailman user. This works fine,
I can send mail to anythinghere@???

But, when sending mail to support@???, Exim rejects the
mail:

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

   root@???
     (ultimately generated from support@???)
     User 0 set for local_delivery transport is on the never_users list:
     retry timeout exceeded

Can anyone tell me what's special about the "support" name? How come it
gets mapped to "root" somehow?

Br,

Morten