cannot send mails to domains on my server

[kreilinger]

hello,

i set up a server (debian etch) with ispcp rc4 (just for testing purposes)

i registered a domain at dyndns.com and opend the needed ports in my router.

everything works fine except reciving emails.

when i try to send an email to an adress on my server i get back an email from the mail delivery system of my server which says:

<addr@domain.ath.cx>: unknown user: "addr"

but this user exists! (i can log into webmail and even check mails with my email client)

anybody able to tell me where the problem is?