[Exim] strange behaviour of relay

hello,

i'm using an exim4 in debian/testing as relay, and configured both relay
and my workstation exim to communicate via smtp-auth. this seems to work
well, and i thought that exim accepts _every_ from: header, hostname,
ipaddress, rcpt, etc. per default if it's beeing used as relay through
smtp
auth with tls.
i'm pretty sure that it uses tls since the logs of relay say:
2004-03-08 01:14:49 1B08Px-0006eZ-CB <= jonas@???
H=pd9e9cf97.dip0.t-ipconnect.de (nazgul.resivo.net) [217.233.207.151]
U=Debian-exim P=asmtp X=TLS-1.0:RSA_ARCFOUR_SHA:16 A=login_server:jonas
S=715 id=20040308001528.GI3836@???

nazgul.resivo.net is my workstation, and this hostname is not resolvable
from the internet (localnet). pd...t-ipconnect.de is the dnsname of my
dynamic ip given from isp. jonas@??? is my email-address,
configured both in mua (mutt), and /etc/email-addresses at my
workstation.

now the problem:
normally the transport of mail works nice, but not always. some mails
are rejected by relay for reasons i don't understand.
for example:
2004-03-08 00:56:19 H=pd9e9cf97.dip0.t-ipconnect.de (nazgul.resivo.net)
[217.233.207.151] U=Debian-exim F=<jonas@???> rejected RCPT
<mejo@???>: relay not permitted

the real problem is definitelly not the RCPT because sometimes mails to
various recipient are accepted and sometimes rejected.

after changing the sys user mailing from at my workstation most new
mails are accepted by relay, but after some testmails, it restarts
rejecting mails for senceless reasons and without any orderliness.

i used the debian standard config, enabling login auth on relay side and
giving no domains/hosts to relay mail for since domains wouldn't me
restictive enough and host doesn't work because of dynamical ip.

do i maybe have to allow dyndns clients manually in config or what did i
miss to get the system messed up like this?

would be great if you could help me.

bye
jonas

ps: please cc to private mail since i'm not able to read all these
lists.