can't receive mail, send ok.

1. Mails sent ok / receive not

We have a linux mail server installed with sendmail and popper.
Inside our net everything works fine.
Mails sent to users outside are transported as well.
But when users outside sent a mail they get the following message:

----- The following addresses had transient non-fatal errors ----

----- Transcript of session follows ----
Warning: message still undelivered after 4 hours
Will keep trying until message is 1 week, 6 days old

Could anybody help?

Thanks in advance

Dipl.-Ing. Markus Goerres
Lehrstuhl Technische Gebaeudeausruestung
Fakultaet Bauwesen
Universitaet Dortmund
Tel: +49-231-755-4777
Fax: +49-231-755-2466
Mobil: +49-171-8069601

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