[exim] smtp auth / forwarding problem

Hi everyone

I have a problem with forwarding addresses and smtp auth

setup:
mailserver1
- virtual users (mysqldb): user1

mailserver2
- /etc/aliases: foo@mailserver2: user1@mailserver1

When I send a mail from user1@mailserver1 to foo@mailserver2 via
mailserver1 (using smtp auth) the mail gets rejected:
(due to setup the mail is actually a mail to self via mailserver2)

2007-08-14 09:26:31 H=mailserver2 [129.132.123.233]
F=<user1@mailserver1> rejected RCPT <user1@mailserver1>: Forged Sender
address (claims to b e local user [user1@mailserver1], but isn't
authenticated)

I noticed that emails with a 'Sender' header (the other mails only have
a 'From' header - e.g. standard thunderbird setup) aren't rejected (i.e.
sender = sth@???)

Other users on mailserver1 can send emails to foo@mailserver2 and I
receive them correctly.

Any ideas where I messed up?

The relevant part of exim.conf:
deny  message          = Sender claims to have a local address, but is
not authenticated nor relayed (try using SMTP-AUTH!)
        log_message     = Forged Sender address (claims to be local
user [${sender_address}], but isn't authenticated)
        !hosts          = +relay_from_hosts
        !authenticated  = *
         condition      = ${if
match_domain{$sender_address_domain}{+local_domains}}

regards
Urs