[exim] Maybe, just maybe its my exim installation?

Hi,

Greeting all you exim-monks of wisdom.

I am very new to the list and would like beg some of your infinite wisdom.

I have been using exim now for the past year and it worked well. Then I
switched my ISP. Now the pppoe connection is no longer directly on the
server.

Past:
MyBox <---pppoe--> ISP1's smtp
Present:
MyBox <---lan---> ADSL Router <---pppoe---> ISP2's smtp

Receiving mail no problem, but when I send mail I get the following
response:

"SMTP error from remote mail server after end of data:
host smtp.isdsl.net [196.26.208.199]: 550 A valid sender header is
required in message (llsjk@???)"

I googled that response and could only find something referencing
callout verification, but suspect it would only matter to verify the
recipients domain. I might be totally wrong there.

My problem is: "Is this at my mail server where I need to make an
adjustment (callout verification or similar.) or do you think that I
have a port forwarding problem on my adsl router?" Should I maybe
switch off the firewall on the router, my box has an iptables firewall
that works well.

If its my exim where would you suggest I start to look?

Any suggestions?

Regards,

Louis