[exim] empty AUTH in MAIL FROM

I have a setup of fetchmail and maildrop that sends some mails (dsns) to a
smarthost, and I need the AUTH field in the MAIL FROM smtp line to be
written and empty. Something like:

MAIL FROM:<> SIZE=1234 AUTH=<>

Is there a way to set this up for the fetchmail user? I have tried to add
the fetchmail user to the trusted_groups, but then either I get no AUTH or
I get AUTH defined.