postfix error message

[mtsouk@freemail.gr]

Dear list,
what do I have to tell postfix in order to correct the following error:

-------------

This is the Postfix program at host debian.

I'm sorry to have to inform you that the message returned
below could not be delivered to one or more destinations.

For further assistance, please send mail to <postmaster>

If you do so, please include this problem report. You can
delete your own text from the message returned below.

                        The Postfix program

<linux-greek-users@lists.hellug.gr>: host tux.hellug.gr[194.219.170.190] said:
    501 5.1.8 Sender domain must exist

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It is a dial-up home system. I cannot send a message even when I am online.

TIA,
Mihalis.


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