One user cannot write Emails to certain domains

[curan]

Hi,

In our company network we are running Exchange 5.5. Everyone, except for one user, can send Emails anywhere else. This one user cannot send Emails to certain domains such as @t-online.de or @web.de. He gets back a non-delivery report from the Exchange server like that:

Your message did not reach some or all of the intended recipients ...

When looking at the system logs i also saw that this one user could not log in properly. There are messages like the following approximately every minute:

COMPANY\username was validated as /o=company/ou=countrycode/cn=Recipients/cn=initials but was unable to log on to /o=company/ou=countrycode/cn=Recipients/cn=username

Some additional info:
Exchange 5.5 SP4
Windows 2000 Terminal SP4

Any ideas what could be the reason for that?

Any help or suggestions are MUCH appreciated.

Thanks,

Chris

 

[curan]

*** EDIT ***
Nobody can send Emails to this domains