[Dovecot] mail from root rejected

I have dovecot mailserver running well. Everything is fine except that mails from root are rejected (some system processes are sending messages by mail using root as sender). In the logfile i get:

Feb 25 10:05:51 Beethoven postfix/qmgr[5228]: 0C7A8652489: from=root@mydomain, size=893, nrcpt=1 (queue active) Feb 25 10:05:51 Beethoven postfix/smtp[5662]: 0C7A8652489: to=hnat@mydomain, relay=smtp.inode.at[62.99.194.14]:25, delay=0.08, delays=0.04/0/0.02/0.01, dsn=5.0.0, status=bounced (host smtp.inode.at[62.99.194.14] said: 550-Verification failed for root@mydomain 550-Unrouteable address 550 Sender verify failed (in reply to MAIL FROM command))

What can i do to have these mails forwarded?

Thanks in advanve for yout tips