[Exim] Auth Plain

Dear folks,

this might be obvious but we have a problem that we
are stuck in:

--- snip ---
host_accept_relay = "/usr/local/exim/local.networks"
host_auth_accept_relay="*"

fixed_plain:
driver = plaintext
public_name = PLAIN
server_condition = ${if
eq{$3}{${lookup{$2}lsearch{/usr/local/exim/auth_users}{$value}fail}}
{yes}{no}}
server_set_id = $2
--- snap ---

We are using Netscape Messenger as the mail client that tries to deliver
mail through
this server. Even if we come from an IP-Address that is specified in
host_accept_relay
Netscape is requesting a username in the preferences. If we give one, it
asks for
a password. Just because Exim is giving "250-AUTH PLAIN" on EHLO, I assume.
What we want is: let the users that may relay according to
host_accept_relay relay and
those who may not ask for a password. I assume that this is some
client-thingie
but is there a way around this?

Thanks for your help i.a.!

Regards,
Sascha

P.S.: The sun is shining!