553 sorry, that domain isn't in my list of allowed rcpthosts

[ResetSystem4]

'morning,

i have a linux server at home and wanted to set it up as a mailserver (mail-gateway) for my network to forward mails to my ISP (mail.isp.com).

everything works fine if i send mail within isp.com, but when i try to send to some other domain the remote mailserver (mail.isp.com) answers the
&quot;>>> RCPT To:<user@otherdomain.com>&quot;
request with
&quot;<<< 553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)&quot;

and sendmail gives me the
&quot;550 5.1.1 <user@otherdomain.com> ... User unknown&quot;
error.

Does anybody have a clue of what's going wrong?
(I am a total beginner, so i probably won't understand sendmail.cf-entries clearly):

Thanks in advance,
RS4